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.