Add cmake check for whether setpgrp takes arguments.

FreeBSD declares setpgrp() as taking two arguments, like Linux's
setpgid(). Detect that at configure time and adjust the call in
Pageant appropriately.

unix/pageant.c

@@ -330,7 +330,11 @@ void pageant_fork_and_print_env(bool retain_tty)
         /* Get out of our previous process group, to avoid being
          * blasted by passing signals. But keep our controlling tty,
          * so we can keep checking to see if we still have one. */
+#if defined HAVE_NULLARY_SETPGRP
         setpgrp();
+#elif defined HAVE_BINARY_SETPGRP
+        setpgrp(0, 0);
+#endif
     } else {
         /* Do that, but also leave our entire session and detach from
          * the controlling tty (if any). */