ps(1): Make '-a' and '-A' always show all processes

When combined with other options affecting the selection of processes,
except for '-X' and '-x', option '-a' would have no effect (and '-A'
would reduce to just '-x'). This was in contradiction with the rule
applying to all other selection options stating that one process is
listed as soon as any of these options has been specified and selects
it, which is both mandated by POSIX and arguably a natural expectation.

MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49617 (code)
Differential Revision: https://reviews.freebsd.org/D49618 (manual page)