Add check if we need to precompose unicode on Mac

This adds initialization of core.precomposeunicode to repo init
on Mac.  This is necessary because when a Mac accesses a repo on
a VFAT or SAMBA file system, it will return directory entries in
decomposed unicode even if the filesystem entry is precomposed.

This also removes caching of a number of repo properties from the
repo init pipeline because these are properties of the specific
filesystem on which the repo is created, not of the system as a
whole.
Put hooks in place for precompose in dirload fn

This doesn't actual do string precompose but it puts the hooks in
place into the iterators and the git_path_dirload function so that
the actual precompose work is ready to go.
Initial iconv hookup for precomposed unicode

This hooks up git_path_direach and git_path_dirload so that they
will take a flag indicating if directory entry names should be
tested and converted from decomposed unicode to precomposed form.
This code will only come into play on the Apple platform and even
then, only when certain types of filesystems are used.

This involved adding a flag to these functions which involved
changing a lot of places in the code.

This was an opportunity to do a bit of code cleanup here and there,
for example, getting rid of the git_futils_cleanupdir_r function in
favor of a simple flag to git_futils_rmdir_r to not remove the top
level entry.  That ended up adding depth tracking during rmdir_r
which led to a safety check for infinite directory recursion.  Yay.

This hasn't actually been tested on the Mac filesystems where the
issue occurs.  I still need to get test environment for that.
Add to Git authors who have agreed to relicense

In email, Torsten Bögershausen agreed that we could use his code
from core Git in libgit2 under the modified license.  Also, since
his work is the basis for much of the precompose unicode support,
I have added him to the AUTHORS file as well.
Simplify git_path_is_empty_dir implementation

This simplifies git_path_is_empty_dir on both Windows (getting rid
of git_buf allocation inside the function) and other platforms (by
just using git_path_direach), and adds tests for the function, and
uses the function to simplify some existing tests.
Wrap iconv stuff and write tests

This adds a simple wrapper around the iconv APIs and uses it
instead of the old code that was inlining the iconv stuff.  This
makes it possible for me to test the iconv logic in isolation.
A "no iconv" version of the API was defined with macros so that
I could have fewer ifdefs in the code itself.
Clean up annoying warnings

The indexer code was generating warnings on Windows 64-bit.  I
looked closely at the logic and was able to simplify it a bit.

Also this fixes some other Windows and Linux warnings.
Update repo init with fewer platform assumptions

The repo init code was assuming Windows == no filemode, and
Mac or Windows == no case sensitivity.  Those assumptions are not
consistently true depending on the mounted file system.  This is a
first step to removing those assumptions.  It focuses on the repo
init code and the tests of that code.  There are still many other
tests that are broken when those assumptions don't hold true, but
this clears up one area of the code.

Also, this moves the core.precomposeunicode logic to be closer to
the current logic in core Git where it will be set to true on any
filesystem where composed unicode is decomposed when read back.
More cleanups to remove WIN assumptions

This cleans up more of the test suite to check actual filesystem
behavior instead of relying on Windows vs. Mac vs. Linux to test.
Add git_repository_reset_filesystem and fix tests

When a repository is transferred from one file system to another,
many of the config settings that represent the properties of the
file system may be wrong.  This adds a new public API that will
refresh the config settings of the repository to account for the
change of file system.  This doesn't do a full "reinitialize" and
operates on a existing git_repository object refreshing the config
when done.

This commit then makes use of the new API in clar as each test
repository is set up.

This commit also has a number of other clar test fixes where we
were making assumptions about the type of filesystem, either based
on outdated config data or based on the OS instead of the FS.
More filemode cleanups for FAT on MacOS

This cleans up some additional issues.  The main change is that
on a filesystem that doesn't support mode bits, libgit2 will now
create new blobs with GIT_FILEMODE_BLOB always instead of being
at the mercy to the filesystem driver to report executable or not.
This means that if "core.filemode" lies and claims that filemode
is not supported, then we will ignore the executable bit from the
filesystem.  Previously we would have allowed it.

This adds an option to the new git_repository_reset_filesystem to
recurse through submodules if desired.  There may be other types
of APIs that would like a "recurse submodules" option, but this
one is particularly useful.

This also has a number of cleanups, etc., for related things
including trying to give better error messages when problems come
up from the filesystem.  For example, the FAT filesystem driver on
MacOS appears to return errno EINVAL if you attempt to write a
filename with invalid UTF-8 in it.  We try to capture that with a
better error message now.
Case sensitivity issues on Linux

A couple of tests were actually dealing incorrectly with case
sensitivity issues on Linux because they were relying on having
core.ignorecase set to true.  Now that the fixture initialization
sets the case sensitivity to be accurate for the platform, it
exposed bugs in these tests.
Make reference lookups apply precomposeunicode

Before these changes, looking up a reference would return the
same precomposed or decomposed form of the reference name that
was used to look it up, so on MacOS which ignores the difference
between the two, a single reference could be looked up either way
and git_reference_name would return the form of the name that was
used to look it up!  This change makes lookup always return the
precomposed name if core.precomposeunicode is set regardless of
which version was used to look it up.  The reference iterator was
already returning the precomposed form from earlier work.

This also updates the CMakeLists.txt rules for enabling iconv
usage because the clar tests for this code were actually not being
activated properly with the old version.

Finally, this moves git_repository_reset_filesystem from include/
git2/repository.h to include/git2/sys/repository.h since it is not
really a function that normal library users should have to think
about very often.
Rename new fn to git_repository_reinit_filesystem
Merge pull request #1886 from libgit2/precompose-utf8

Add support for core.precomposeunicode on Mac