Avoiding concurrent calls to bindtextdomain()

According to the discussion in [1], it's not as safe as we supposed
to allow different threads to call bindtextdomain() concurrently.
Here is a patchset to prevent that by acquiring a mutex around
the libpq and ecpglib calls that are at risk.

In libpq, this would've required yet a third copy of the
Windows-specific ugliness in default_threadlock() and pgtls_init().
I didn't particularly want to do that, so I stole some ideas
from ecpglib to create a more POSIX-compliant emulation of
pthread_mutex_lock(). 0001 attached is the refactoring needed
to make that happen, and then 0002 is the actual bug fix.

0001 also gets rid of the possibility that pthread_mutex_init/
pthread_mutex_lock could fail due to malloc failure. This seems
important since default_threadlock() assumes that pthread_mutex_lock
cannot fail in practice. I observe that ecpglib also assumes that,
although it's using CreateMutex() which has documented failure
conditions. So I wonder if we ought to copy this implementation
back into ecpglib; but I've not done that here.

regards, tom lane

[1] https://www.postgresql.org/message-id/flat/18312-bbbabc8113592b78%40postgresql.org