process startup: Rename postmaster's --forkboot to --forkaux.
It is confusing that aux processes are started with --forkboot, given that
bootstrap mode cannot run below postmaster.
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