Use our own getopt() on OpenBSD.

Recent OpenBSD (at least 5.9 and up) has a version of getopt(3)
that will not cope with the "-:" spec we use to accept double-dash
options in postgres.c and postmaster.c.  Admittedly, that's a hack
because POSIX only requires getopt() to allow alphanumeric option
characters.  I have no desire to find another way, however, so
let's just do what we were already doing on Solaris: force use
of our own src/port/getopt.c implementation.

In passing, improve some of the comments around said implementation.

Per buildfarm and local testing.  Back-patch to all supported branches.

Discussion: https://postgr.es/m/30197.1547835700@sss.pgh.pa.us

src/include/pg_getopt.h

@@ -1,4 +1,13 @@
 /*
+ * Postgres files that use getopt(3) always include this file.
+ * We must cope with three different scenarios:
+ * 1. We're using the platform's getopt(), and we should just import the
+ *    appropriate declarations.
+ * 2. The platform lacks getopt(), and we must declare everything.
+ * 3. The platform has getopt(), but we're not using it because we don't
+ *    like its behavior.  The declarations we make here must be compatible
+ *    with both the platform's getopt() and our src/port/getopt.c.
+ *
  * Portions Copyright (c) 1987, 1993, 1994
  * The Regents of the University of California.  All rights reserved.
  *
@@ -39,6 +48,7 @@ extern int	optopt;
 extern int	optreset;
 #endif
 
+/* Provide getopt() declaration if the platform doesn't have it */
 #ifndef HAVE_GETOPT
 extern int	getopt(int nargc, char *const *nargv, const char *ostr);
 #endif