Break out OpenSSL-specific code to separate files.

This refactoring is in preparation for adding support for other SSL
implementations, with no user-visible effects. There are now two #defines,
USE_OPENSSL which is defined when building with OpenSSL, and USE_SSL which
is defined when building with any SSL implementation. Currently, OpenSSL is
the only implementation so the two #defines go together, but USE_SSL is
supposed to be used for implementation-independent code.

The libpq SSL code is changed to use a custom BIO, which does all the raw
I/O, like we've been doing in the backend for a long time. That makes it
possible to use MSG_NOSIGNAL to block SIGPIPE when using SSL, which avoids
a couple of syscall for each send(). Probably doesn't make much performance
difference in practice - the SSL encryption is expensive enough to mask the
effect - but it was a natural result of this refactoring.

Based on a patch by Martijn van Oosterhout from 2006. Briefly reviewed by
Alvaro Herrera, Andreas Karlsson, Jeff Janes.

src/backend/postmaster/fork_process.c

@@ -17,7 +17,7 @@
 #include <sys/stat.h>
 #include <sys/time.h>
 #include <unistd.h>
-#ifdef USE_SSL
+#ifdef USE_OPENSSL
 #include <openssl/rand.h>
 #endif
 
@@ -110,7 +110,7 @@ fork_process(void)
 		/*
 		 * Make sure processes do not share OpenSSL randomness state.
 		 */
-#ifdef USE_SSL
+#ifdef USE_OPENSSL
 		RAND_cleanup();
 #endif
 	}