BUG/MEDIUM: auth/threads: use of crypt() is not thread-safe

BUG/MEDIUM: auth/threads: use of crypt() is not thread-safe

It was reported here that authentication may fail when threads are
enabled :

    https://bugzilla.redhat.com/show_bug.cgi?id=1643941

While I couldn't reproduce the issue, it's obvious that there is a
problem with the use of the non-reentrant crypt() function there.
On Linux systems there's crypt_r() but not on the vast majority of
other ones. Thus a first approach consists in placing a lock around
this crypt() call. Another patch may relax it when crypt_r() is
available.

This fix must be backported to 1.8. Thanks to Ryan O'Hara for the
quick notification.