Remove the ability to select random number generator independently from
SSL library. Instead, use the random number generator from the SSL
library (today only OpenSSL supported) if one is configured. If no SSL
library is configured, use the platform default (which means use
CryptoAPI on Win32 and /dev/urandom on Linux).

This also restructures pg_strong_random.c to have three clearly separate
sections, one for each implementation, with two functions in each,
instead of a scattered set of ifdefs throughout the whole file.

Author: Daniel Gustafsson, Magnus Hagander, Michael Paquier
Discussion: https://postgr.es/m/632623.1605460616@sss.pgh.pa.us