LibreSSL now supports loading of CA certificates from memory, replace

LibreSSL now supports loading of CA certificates from memory, replace
the internal and long-serving ssl_ctx_load_verify_memory() function
with a call to the SSL_CTX_load_verify_mem() API function.  The
ssl_privsep.c file with hacks for using OpenSSL in privsep'ed
processes can now go away; portable versions of smtpd and relayd
should start depending on LibreSSL or they have to carry ssl_privsep.c
in openbsd-compat to work with legacy OpenSSL.  No functional change.

Based on previous discussions with gilles@ bluhm@ and many others
OK bluhm@ (as part of the libcrypto/libssl/libtls diff)