add -F/--force option to help/usage

Index: autofs/daemon/automount.c
===================================================================
@@ -1678,6 +1678,7 @@ static void usage(void)
 		"			set daemon log verbosity\n"
 		"	-C --dont-check-daemon\n"
 		"			don't check if daemon is already running\n"
+		"	-F --force	force unlink of mount tree at startup\n"
 		"	-V --version	print version, build config and exit\n"
 		, program);
 }