Use RAND_poll() for seeding randomness after fork().
OpenSSL deprecated RAND_cleanup(), and OpenSSL 1.1.0 made it into a
no-op. Replace it with RAND_poll(), per an OpenSSL community
recommendation. While this has no user-visible consequences under
OpenSSL defaults, it might help under non-default settings.
Daniel Gustafsson, reviewed by David Steele and Michael Paquier.
Discussion: https://postgr.es/m/
9B038FA5-23E8-40D0-B932-
D515E1D8F66A@yesql.se