Default to dynamic_shared_memory_type=sysv on Solaris.

Default to dynamic_shared_memory_type=sysv on Solaris.

POSIX shm_open() can sleep for a long time and fail spuriously because
of contention on an internal lock file on Solaris (and presumably
illumos).  Commit 389869af fixed the main problem with this, namely that
we could crash, but it's now clear that "posix" is not a good default.

Therefore, choose "sysv" at initdb time on Solaris and illumos.  Other
choices are still available by editing the postgresql.conf file.

Back-patch only to 15, because contention is much less likely further
back, and it doesn't seem like a good idea to change this in released
branches.  This should clear up the failures on build farm animal
margay.

Discussion: https://postgr.es/m/CA%2BhUKGKqKrCV5xKWfh9rnm%3Do%3DDwZLTLtnsj_XpUi9g5%3DV%2B9oyg%40mail.gmail.com