Add -d option to pg_basebackup and pg_receivexlog, for connection string.

Without this, there's no way to pass arbitrary libpq connection parameters
to these applications. It's a bit strange that the option is called
-d/--dbname, when in fact you can *not* pass a database name in it, but it's
consistent with other client applications where a connection string is also
passed using -d.

Original patch by Amit Kapila, heavily modified by me.

src/bin/pg_basebackup/streamutil.h

@@ -1,6 +1,7 @@
 #include "libpq-fe.h"
 
 extern const char *progname;
+extern char *connection_string;
 extern char *dbhost;
 extern char *dbuser;
 extern char *dbport;