Receiver now rejects invalid filenames in filelist.

If the receiver gets a filename with a leading slash (w/o --relative)
and/or a filename with an embedded ".." dir in the path, it dies with
an error (rather than continuing). Those invalid paths should never
happen in reality, so just reject someone trying to pull a fast one.
Avoid infinite wait reading secrets file.
Expand the backslash description a bit more in excludes.
Add new-style compression that skips matching data.

Adding new-style compression that only compresses the literal data that
is sent over the wire and not also matching file data that was not sent.
This new-style compression is compatible with external zlib instances,
and will eventually become the default (once enough time has passed that
all servers support the --new-compress and --old-compress options).

NOTE: if you build rsync with an external zlib (i.e. if you specified
configure --with-included-zlib=no) you will ONLY get support for the
--new-compress option!  A client will treat -z as uncompressed (with a
warning) and a server will exit with an error (unless -zz was used).
Change args to file_checksum() to prepare for future changes.
Check F_IS_ACTIVE() in a few more spots.

The code needs to ignore flist entries that were marked inactive (by
either duplicate removal or empty-dir pruning) in a few more spots.
Fix usermap/groupmap parsing of MIN-MAX IDs.
Check for attr lib.