pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/99175141c9254318e5894ac30b9fdb622612acda

Modified Files
--------------
doc/src/sgml/ref/pg_dump.sgml                 |  2 ++
doc/src/sgml/ref/pg_dumpall.sgml              |  4 +++-
doc/src/sgml/ref/pg_restore.sgml              |  7 ++++++-
src/bin/pg_archivecleanup/pg_archivecleanup.c |  2 +-
src/bin/pg_dump/pg_backup_archiver.c          |  3 ++-
src/bin/pg_dump/pg_dump.c                     |  2 +-
src/bin/pg_dump/pg_dumpall.c                  |  2 +-
src/bin/pg_dump/pg_restore.c                  |  2 +-
src/bin/pg_rewind/pg_rewind.c                 |  2 +-
src/bin/pgbench/pgbench.c                     |  2 +-
src/common/logging.c                          | 18 ++++++++++++++++++
src/include/common/logging.h                  |  1 +
12 files changed, 38 insertions(+), 9 deletions(-)