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
------
master

Details
-------
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(-)