Fix pgbench's failure to honor the documented long-form option "--builtin".

Not only did it not accept --builtin as a synonym for -b, but what it did
accept as a synonym was --tpc-b (huh?), which it got even further wrong
by marking as no_argument, so that if you did try that you got a core
dump.  I suppose this is leftover from some early design for the new
switches added by commit 8bea3d22, but it's still pretty sloppy work.

Per bug #14580 from Stepan Pesternikov.  Back-patch to 9.6 where the
error was introduced.

Report: https://postgr.es/m/20170307123347.25054.73207@wrigleys.postgresql.org

src/bin/pgbench/pgbench.c

@@ -3610,7 +3610,7 @@ main(int argc, char **argv)
 {
 	static struct option long_options[] = {
 		/* systematic long/short named options */
-		{"tpc-b", no_argument, NULL, 'b'},
+		{"builtin", required_argument, NULL, 'b'},
 		{"client", required_argument, NULL, 'c'},
 		{"connect", no_argument, NULL, 'C'},
 		{"debug", no_argument, NULL, 'd'},