pgsql: process startup: Split single user code out of PostgresMain().

process startup: Split single user code out of PostgresMain().

It was harder than necessary to understand PostgresMain() because the code for
a normal backend was interspersed with single-user mode specific code. Split
most of the single-user mode code into its own function
PostgresSingleUserMain(), that does all the necessary setup for single-user
mode, and then hands off after that to PostgresMain().

There still is some single-user mode code in InitPostgres(), and it'd likely
be worth moving at least some of it out. But that's for later.

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7c83a3bf51489e5b48c567c2ac54fed030d23c52

Modified Files
--------------
src/backend/main/main.c | 5 +-
src/backend/postmaster/postmaster.c | 8 +-
src/backend/tcop/postgres.c | 152 ++++++++++++++++++++----------------
src/include/tcop/tcopprot.h | 5 +-
4 files changed, 90 insertions(+), 80 deletions(-)