pgsql: process startup: Separate out BootstrapModeMain from AuxiliaryPr

process startup: Separate out BootstrapModeMain from AuxiliaryProcessMain.

There practically was no shared code between the two, once all the ifs are
removed. And it was quite confusing that aux processes weren't actually
started by the call to AuxiliaryProcessMain() in main().

There's more to do, AuxiliaryProcessMain() should move out of bootstrap.c, and
BootstrapModeMain() shouldn't use/be part of AuxProcType.

Author: Andres Freund <andres(at)anarazel(dot)de>
Reviewed-By: Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>
Reviewed-By: Robert Haas <robertmhaas(at)gmail(dot)com>
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/5aa4a9d2077fa902b4041245805082fec6be0648

Modified Files
--------------
src/backend/bootstrap/bootstrap.c | 324 ++++++++++++++++++------------------
src/backend/main/main.c | 2 +-
src/backend/postmaster/postmaster.c | 43 ++---
src/include/bootstrap/bootstrap.h | 4 +-
4 files changed, 187 insertions(+), 186 deletions(-)