pgcrypto:  fix memset() calls that might be optimized away

Specifically, on-stack memset() might be removed, so:

	* Replace memset() with px_memset()
	* Add px_memset to copy_crlf()
	* Add px_memset to pgp-s2k.c

Patch by Marko Kreen

Report by PVS-Studio

Backpatch through 8.4.