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/include/pg_config_manual.h

@@ -144,6 +144,15 @@
 #define USE_PREFETCH
 #endif
 
+/*
+ * USE_SSL code should be compiled only when compiling with an SSL
+ * implementation.  (Currently, only OpenSSL is supported, but we might add
+ * more implementations in the future.)
+ */
+#ifdef USE_OPENSSL
+#define USE_SSL
+#endif
+
 /*
  * This is the default directory in which AF_UNIX socket files are
  * placed.  Caution: changing this risks breaking your existing client