SHELLERS for SYSCF, MacOS 10.5 hints file (trunk only)

Add SHELLERS - people allowed to use ! command with same syntax as WIZARDS.
Add new hints file for 10.5, since the rules and commands for groups changed
(new commands introduced in 10.4, old ones removed in 10.5; creating a new
user under 10.4 gave you a matching group, in 10.5 it doesn't).  Also move
shared build into roughly right place in file system when being installed
for root - don't use ~root.
Makefile.top - don't remove ./-p unless it exists (that's always annoyed me).
fix error invoking macosx.sh

sys/unix/unixunix.c

@@ -244,6 +244,15 @@ int
 dosh()
 {
 	register char *str;
+# ifdef SYSCF
+	if(   !sysopt.shellers
+	   || !sysopt.shellers[0]
+	   || !check_user_string(sysopt.shellers)
+	){
+		Norep("Unknown command '!'.");
+		return 0;
+	}
+# endif
 	if(child(0)) {
 		if((str = getenv("SHELL")) != (char*)0)
 			(void) execl(str, str, (char *)0);