Clear the error message on git_libgit2_shutdown for all versions of
the library (no threads and Win32 threads). Drop the giterr_clear
in clar, as that shouldn't be necessary.
This describes their purpose better, as we now initialize ssl and some
other global stuff in there. Calling the init function is not something
which has been optional for a while now.
There are a number of tests that modify the global or system
search paths during the tests. This adds a helper function to
make it easier to restore those paths and makes sure that they
are getting restored in a manner that preserves test isolation.
