* audit: add comment about false positive valgrind warning
* xauth: crypt() can return NULL (ie in FIPS mode)
* threads: protect crypt() with a mutex

crypt_r requires -D_GNU_SOURCE. Not sure crypt_r is implemented under
OpenBSD and FreeBSD.  crypt requires -D_XOPEN_SOURCE and thus should
be implemented on every Unix/Unix-like. The pthread library is even
implemented under Windows/Cygwin. It is implemented on Linux/HP-UX/Tru64
(both HP's Unix). So the pthread library should as well be under
OpenBSD/FreeBSD.

Patch by Philippe Vouters <philippe.vouters@laposte.net>

Signed-off-by: Paul Wouters <pwouters@redhat.com>