libpq: Add PQgetThreadLock() to mirror PQregisterThreadLock()

Allow libpq clients to retrieve the current pg_g_threadlock pointer with
PQgetThreadLock(). Single-threaded applications could already do this in
a convoluted way:

    pgthreadlock_t tlock;

    tlock = PQregisterThreadLock(NULL);
    PQregisterThreadLock(tlock);    /* re-register the callback */

    /* use tlock */

But a generic library can't do that without potentially breaking
concurrent libpq connections.

The motivation for doing this now is the libpq-oauth plugin, which
currently relies on direct injection of pg_g_threadlock, and should
ideally not.

Reviewed-by: Zsolt Parragi <[email protected]>
Discussion: 
https://postgr.es/m/CAOYmi%2BmEU_q9sr1PMmE-4rLwFN%3DOjyndDwFZvpsMU3RNJLrM9g%40mail.gmail.com
Discussion: 
https://postgr.es/m/CAOYmi%2B%3DMHD%2BWKD4rsTn0v8220mYfyLGhEc5EfhmtqrAb7SmC5g%40mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/b8d76858353e40f888c809eb14a974cf13bf23ab

Modified Files
--------------
doc/src/sgml/libpq.sgml           |  6 +++++-
src/interfaces/libpq/exports.txt  |  1 +
src/interfaces/libpq/fe-connect.c |  7 +++++++
src/interfaces/libpq/libpq-fe.h   | 10 ++++++++--
4 files changed, 21 insertions(+), 3 deletions(-)

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