pgcrypto already supports key-stretching during symmetric encryption,
including the salted-and-iterated method; but the number of iterations
was not configurable. This commit implements a new s2k-count parameter
to pgp_sym_encrypt() which permits selecting a larger number of
iterations.
Author: Jeff Janes
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.