Remove libpq's use of abort(3) to handle mutex failure cases.

Doing an abort() seems all right in development builds, but not in
production builds of general-purpose libraries.  However, the functions
that were doing this lack any way to report a failure back up to their
callers.  It seems like we can just get away with ignoring failures in
production builds, since (a) no such failures have been reported in the
dozen years that the code's been like this, and (b) failure to enforce
mutual exclusion during fe-auth.c operations would likely not cause any
problems anyway in most cases.  (The OpenSSL callbacks that use this
macro are obsolete, so even less likely to cause interesting problems.)

Possibly a better answer would be to break compatibility of the
pgthreadlock_t callback API, but in the absence of field problem
reports, it doesn't really seem worth the trouble.

Discussion: https://postgr.es/m/3131385.1624746109@sss.pgh.pa.us

src/interfaces/libpq/fe-connect.c

@@ -7254,6 +7254,11 @@ pqGetHomeDirectory(char *buf, int bufsize)
 /*
  * To keep the API consistent, the locking stubs are always provided, even
  * if they are not required.
+ *
+ * Since we neglected to provide any error-return convention in the
+ * pgthreadlock_t API, we can't do much except Assert upon failure of any
+ * mutex primitive.  Fortunately, such failures appear to be nonexistent in
+ * the field.
  */
 
 static void
@@ -7273,7 +7278,7 @@ default_threadlock(int acquire)
 		if (singlethread_lock == NULL)
 		{
 			if (pthread_mutex_init(&singlethread_lock, NULL))
-				PGTHREAD_ERROR("failed to initialize mutex");
+				Assert(false);
 		}
 		InterlockedExchange(&mutex_initlock, 0);
 	}
@@ -7281,12 +7286,12 @@ default_threadlock(int acquire)
 	if (acquire)
 	{
 		if (pthread_mutex_lock(&singlethread_lock))
-			PGTHREAD_ERROR("failed to lock mutex");
+			Assert(false);
 	}
 	else
 	{
 		if (pthread_mutex_unlock(&singlethread_lock))
-			PGTHREAD_ERROR("failed to unlock mutex");
+			Assert(false);
 	}
 #endif
 }