Add pg_strong_random_init function to initialize random number generator

Add pg_strong_random_init function to initialize random number generator

Currently only OpenSSL requires this initialization, but in the future
other SSL implementations are likely to need it as well. Abstracting
this functionality out into a separate function makes this cleaner and
more clear, and also removes the dependency on OpenSSL headers from
fork_process.c.

OpenSSL is special in that we need to initialize this random number
generator even if we're not going to use it directly, until we drop
support for everything prior to OpenSSL 1.1.1. (And of course also if we
actually use it). All other implementations are left empty at this time,
but more are expected to be added in the future.

Author: Daniel Gustafsson <daniel@yesql.se>, Michael Paquier <michael@paquier.xyz>
Reviewed-By: Magnus Hagander <magnus@hagander.net>
Discussion: https://postgr.es/m/F6291C3C-747C-4C93-BCE0-28BB420B1FF5@yesql.se