The compiler was generating a bunch of warnings for
git_mutex_init and git_mutex_lock when GIT_THREADS
was not defined (i.e. when not using -DTHREADSAFE=ON).

Also remove an unused variable from tests/path/core.c.

Move the definition of git_thread_yield() to the test which needs it and
add the correct definition for it for FreeBSD and derivatives.

Original patch adding FreeBSD and derivatives by @jacquesg.

This makes the lock management on the index a little bit broader,
having a number of routines hold the lock across looking up the
item to be modified and actually making the modification.  Still
not true thread safety, but more pure index modifications are now
safe which allows the simple cases (such as starting up a diff
while index modifications are underway) safe enough to get the
snapshot without hitting allocation problems.

As part of this, I simplified the allocation of index entries to
use a flex array and just put the path at the end of the index
entry.  This makes every entry self-contained and makes it a
little easier to feel sure that pointers to strings aren't
being accidentally copied and freed while other references are
still being held.

This loads SRWLock APIs at runtime and in their absence (i.e. on
Windows before Vista) falls back on a regular CRITICAL_SECTION
that will not permit concurrent readers.

This makes libgit2 require Windows Vista or newer if it is going
to be compiled with the THREADSAFE option

This is the first use we have of pthread_rwlock_t in libgit2.
Hopefully it won't cause any serious portability problems.