Detect encrypted archive entries (ZIP, RAR, 7Zip)

With this change you can detect if an archive entry is encrypted. The
archive formats covered with this change are: ZIP, RAR, and 7zip. Other
formats can be added quite simply by looking at the already supported
formats. For all the already supported formats we have tests that check
if:

	* an archive entries's data is encryped (data test)
	* an archive entries's metadata is encrypted (header test)
	* one file is encrypted while another is not (partially test)

These new functions are introduced.

int archive_read_format_capabilities(struct archive*)

	Returns a bitmask of capabilities that are supported by the archive
	format reader. If the reader has no special capabilities,
	ARCHIVE_READ_FORMAT_CAPS_NONE is returned; otherwise 0 is returned.

	You can call this function even before reading the first header from
	an archive.

	Return Values:
	* ARCHIVE_READ_FORMAT_CAPS_ENCRYPT_DATA
		The reader supports detection of encrypted data.
	* ARCHIVE_READ_FORMAT_CAPS_ENCRYPT_METADATA
		The reader supports detection of encrypted metadata (e.g.
		filename, modification time, size, etc.)
	* ARCHIVE_READ_FORMAT_CAPS_NONE
		None of the above capabilities. If this value is returned, this
		doesn't mean that the format itself doesn't support any type of
		encryption it simply means that the reader is not capable of
		detecting it.

int archive_read_has_encrypted_entries(struct archive *)

	Returns "true" (non-zero) if the archive contains at least one
	encrypted entry, no matter which encryption type (data or metadata)
	is used; otherwise 0 is returned.

	You should only call this function after reading the first header
	from an archive.

	NOTE: I'm not sure that this function will stay in for long.

int archive_entry_is_data_encrypted(struct archive_entry*)

	Returns "true" (non-zero) if the archive entry's data is encrypted;
	otherwise 0 is returned. You can call this function after calling
	archive_read_next_header().

int archive_entry_is_metadata_encrypted(struct archive_entry*)

	Returns "true" (non-zero) if the archive entry's metadata is
	encrypted; otherwise 0 is returned. You can call this function after
	calling archive_read_next_header().

int archive_entry_is_encrypted(struct archive_entry*)

	Returns "true" (non-zero) if either the archive entry's data and/or
	it's metadata is encrypted; otherwise 0 is returned. You can call
	this function after calling archive_read_next_header().

If you use archive_read_format_capabilities() in combination with one of
the archive_entry_is_[data|metadata]_encrypted() functions, you can be
sure that you've encountered an encrypted entry. This allows you to
react differently depending on libarchive's return codes. For instance,
you might want to skip encrypted files from being extracted until
decryption support has been implemented.

Here's how I generated the 7zip test files:
-------------------------------------------

With header encrpytion (-mhe=on):

    $ rm -f test_read_format_7zip_encryption_header.7z
    $ echo "foo" > bar.txt && 7z a -mhe=on -p12345678 \
    test_read_format_7zip_encryption_header.7z bar.txt

    $ uuencode test_read_format_7zip_encryption_header.7z \
    test_read_format_7zip_encryption_header.7z > \
    test_read_format_7zip_encryption_header.7z.uu

Without header encrpytion (-mhe=off):

    $ rm -f test_read_format_7zip_encryption.7z
    $ echo "foo" > bar.txt && 7z a -mhe=off -p12345678 \
    test_read_format_7zip_encryption.7z bar.txt

    $ uuencode test_read_format_7zip_encryption.7z \
    test_read_format_7zip_encryption.7z > \
    test_read_format_7zip_encryption.7z.uu

Partially encrypted archive:

	$ rm -f test_read_format_7zip_encryption_partially.7z
	$ echo "foo" > bar_unencrypted.txt && 7z a \
	test_read_format_7zip_encryption_partially.7z bar_unencrypted.txt
	$ echo "foo" > bar_encrypted.txt && 7z a -mhe=off -p12345678 \
	test_read_format_7zip_encryption_partially.7z bar_encrypted.txt

	$ uuencode test_read_format_7zip_encryption_partially.7z \
	test_read_format_7zip_encryption_partially.7z > \
	test_read_format_7zip_encryption_partially.7z.uu

Here's how I generated the RAR test files:
------------------------------------------

These are the files we can will add to the archives:

    echo "data of foo.txt" > foo.txt
    echo "data of bar.txt" > bar.txt

 With header encrpytion (-hp):

    rm -f test_read_format_rar_encryption_header.rar
    rar a -hp12345678 test_read_format_rar_encryption_header.rar \
    foo.txt bar.txt

    uuencode test_read_format_rar_encryption_header.rar \
    test_read_format_rar_encryption_header.rar > \
    test_read_format_rar_encryption_header.rar.uu

Without header encrpytion (-p):

    rm -f test_read_format_rar_encryption_data.rar
    rar a -p12345678 test_read_format_rar_encryption_data.rar \
    foo.txt bar.txt

    uuencode test_read_format_rar_encryption_data.rar \
    test_read_format_rar_encryption_data.rar > \
    test_read_format_rar_encryption_data.rar.uu

Partially encrypted archive (-p on "foo.txt" and no password on "bar.txt"):

    rm -f test_read_format_rar_encryption_partially.rar
    rar a -p12345678 test_read_format_rar_encryption_partially.rar foo.txt
    rar a test_read_format_rar_encryption_partially.rar bar.txt

    uuencode test_read_format_rar_encryption_partially.rar \
    test_read_format_rar_encryption_partially.rar > \
    test_read_format_rar_encryption_partially.rar.uu

Here's how I generated the ZIP test files:
------------------------------------------

This is how I've created the test files:

These are the files we will add to the archives:
  On Windows:
    echo "data of foo.txt" > foo.txt
    echo "data of bar.txt" > bar.txt

For the creation of the Zip archives I've used the
PKZIP Command Line Add-On available from here:

http://comm.pkware.com/pkzip-cli-download.html

With header (aka central directory) encrpytion (-cd):

  On Windows:
    del /F test_read_format_zip_encryption_header.zip
    pkzipc.exe -add -cryptalgorithm=AES,256 -passphrase=12345678 -cd test_read_format_zip_encryption_header.zip foo.txt bar.txt

  On Linux:
    uuencode test_read_format_zip_encryption_header.zip \
    test_read_format_zip_encryption_header.zip > \
    test_read_format_zip_encryption_header.zip.uu

Without header encrpytion:

  On Windows:
    del /F test_read_format_zip_encryption_data.zip
    pkzipc.exe -add -cryptalgorithm=AES,256 -passphrase=12345678 test_read_format_zip_encryption_data.zip foo.txt bar.txt

  On Linux:
    uuencode test_read_format_zip_encryption_data.zip \
    test_read_format_zip_encryption_data.zip > \
    test_read_format_zip_encryption_data.zip.uu

Partially encrypted archive ("foo.txt" is encrypted and "bar.txt" is not):

  On Windows:
    del /F test_read_format_zip_encryption_partially.zip
    pkzipc.exe -add -cryptalgorithm=AES,256 -passphrase=12345678 test_read_format_zip_encryption_partially.zip foo.txt
    pkzipc.exe -add test_read_format_zip_encryption_partially.zip bar.txt

  On Linux:
    uuencode test_read_format_zip_encryption_partially.zip \
    test_read_format_zip_encryption_partially.zip > \
    test_read_format_zip_encryption_partially.zip.uu