Avoid concurrent calls to bindtextdomain().

Avoid concurrent calls to bindtextdomain().

We previously supposed that it was okay for different threads to
call bindtextdomain() concurrently (cf. commit 1f655fdc3).
It now emerges that there's at least one gettext implementation
in which that triggers an abort() crash, so let's stop doing that.
Add mutexes guarding libpq's and ecpglib's calls, which are the
only ones that need worry about multithreaded callers.

Note: in libpq, we could perhaps have piggybacked on
default_threadlock() to avoid defining a new mutex variable.
I judge that not terribly safe though, since libpq_gettext could
be called from code that is holding the default mutex.  If that
were the first such call in the process, it'd fail.  An extra
mutex is cheap insurance against unforeseen interactions.

Per bug #18312 from Christian Maurer.  Back-patch to all
supported versions.

Discussion: https://postgr.es/m/18312-bbbabc8113592b78@postgresql.org
Discussion: https://postgr.es/m/264860.1707163416@sss.pgh.pa.us