pgsql: Unify several ways to tracking backend type

Unify several ways to tracking backend type

Add a new global variable MyBackendType that uses the same BackendType
enum that was previously only used by the stats collector.  That way
several duplicate ways of checking what type a particular process is
can be simplified.  Since it's no longer just for stats, move to
miscinit.c and rename existing functions to match the expanded
purpose.

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/8e8a0becb335529c66a9f82f88e1419e49b458ae

Modified Files
--------------
src/backend/bootstrap/bootstrap.c   |  48 +++++++----------
src/backend/postmaster/autovacuum.c |   8 +--
src/backend/postmaster/bgworker.c   |   2 +-
src/backend/postmaster/pgarch.c     |   6 +--
src/backend/postmaster/pgstat.c     | 105 ++----------------------------------
src/backend/postmaster/postmaster.c |   7 ++-
src/backend/postmaster/syslogger.c  |   3 +-
src/backend/utils/adt/pgstatfuncs.c |   2 +-
src/backend/utils/init/miscinit.c   |  55 +++++++++++++++++++
src/backend/utils/misc/ps_status.c  |  11 +++-
src/include/miscadmin.h             |  22 ++++++++
src/include/pgstat.h                |  21 +-------
12 files changed, 126 insertions(+), 164 deletions(-)