process startup: Separate out BootstrapModeMain from AuxiliaryProcessMain.

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@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