pgsql: process startup: Move AuxiliaryProcessMain into its own file.

process startup: Move AuxiliaryProcessMain into its own file.

After the preceding commits the auxprocess code is independent from
bootstrap.c - so a dedicated file seems less confusing.

Author: Andres Freund <andres@anarazel.de>
Reviewed-By: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Reviewed-By: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/20210802164124.ufo5buo4apl6yuvs@alap3.anarazel.de

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/0a692109dcc73178962069addf7478ac89950e4d

Modified Files
--------------
src/backend/bootstrap/bootstrap.c   | 157 -----------------------------
src/backend/postmaster/Makefile     |   1 +
src/backend/postmaster/auxprocess.c | 194 ++++++++++++++++++++++++++++++++++++
src/backend/postmaster/postmaster.c |   2 +-
src/include/bootstrap/bootstrap.h   |   2 -
src/include/miscadmin.h             |   2 +-
src/include/postmaster/auxprocess.h |  20 ++++
7 files changed, 217 insertions(+), 161 deletions(-)