Improve common/logging.c's support for multiple verbosity levels.

 Improve common/logging.c's support for multiple verbosity levels.
 
 Instead of hard-wiring specific verbosity levels into the option
 processing of client applications, invent pg_logging_increase_verbosity()
 and encourage clients to implement --verbose by calling that.  Then,
 the common convention that more -v's gets you more verbosity just works.
 
 In particular, this allows resurrection of the debug-grade messages that
 have long existed in pg_dump and its siblings.  They were unreachable
 before this commit due to lack of a way to select PG_LOG_DEBUG logging
 level.  (It appears that they may have been unreachable for some time
 before common/logging.c was introduced, too, so I'm not specifically
 blaming cc8d41511 for the oversight.  One reason for thinking that is
 that it's now apparent that _allocAH()'s message needs a null-pointer
 guard.  Testing might have failed to reveal that before 96bf88d52.)
 
 Discussion: https://postgr.es/m/1173106.1600116625@sss.pgh.pa.us