(ITS#8638) Recursive mutex in libldap_r

[okuznik@symas.com]

Full_Name: Ondrej Kuznik
Version: master
OS: 
URL: ftp://ftp.openldap.org/incoming/Ondrej-Kuznik-20170414-libldap_r-recursive-mutex.patch
Submission from: (NULL) (2a02:c7f:2247:ec00:a3e:8eff:fe52:dac5)


Integrating with libevent, I would find it useful if libldap_r provided a
recursive mutex as well, since this is what libevent uses.

Attached is an implementation. Platforms included:
- POSIX (the only one I could test properly)
- Win NT (regular mutexes, since they're recursive already, according to the
docs)
- GNU pth (same as NT, documentation says that all mutexes are recursive)
- stub (noop)

Not included:
- thr_thr.c - not sure what that is
- thr_cthreads.c - does not seem to support a recursive thread implementation

I'm not sure who still uses the older platforms, I suppose only POSIX and Win NT
are being used in the wild? Not that any users should be impacted by this since
it's not referenced anywhere in the project yet.