pgsql: Add backend type to csvlog and optionally log_line_prefix

Add backend type to csvlog and optionally log_line_prefix

The backend type, which corresponds to what
pg_stat_activity.backend_type shows, is added as a column to the
csvlog and can optionally be added to log_line_prefix using the new %b
placeholder.

Reviewed-by: Julien Rouhaud <rjuju123@gmail.com>
Reviewed-by: Kuntal Ghosh <kuntalghosh.2007@gmail.com>
Reviewed-by: Alvaro Herrera <alvherre@2ndquadrant.com>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://www.postgresql.org/message-id/flat/c65e5196-4f04-4ead-9353-6088c19615a3@2ndquadrant.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/70a7b4776be4e776221e578d8ca53b2b9f8118c0

Modified Files
--------------
doc/src/sgml/config.sgml                      | 20 ++++++++++++++++++-
src/backend/utils/error/elog.c                | 28 +++++++++++++++++++++++++++
src/backend/utils/misc/postgresql.conf.sample |  1 +
src/test/regress/pg_regress.c                 |  2 +-
4 files changed, 49 insertions(+), 2 deletions(-)