Add f_isset() utility subroutine for checking if a variable is set but in

a more readable fashion.

usr.sbin/bsdconfig/share/common.subr

@@ -195,6 +195,16 @@ f_getvar()
 	return $__retval
 }
 
+# f_isset $var
+#
+# Check if variable $var is set. Returns success if variable is set, otherwise
+# returns failure.
+#
+f_isset()
+{
+	eval [ \"\${${1%%[$IFS]*}+set}\" ]
+}
+
 # f_die [ $status [ $fmt [ $opts ... ]]]
 #
 # Abruptly terminate due to an error optionally displaying a message in a

usr.sbin/bsdconfig/startup/rcdelete

@@ -388,7 +388,7 @@ while :; do
 		var="${mtag# }"
 
 		# Toggle the state-variable and loop back to menu
-		if eval [ \"\${_${var}_delete+set}\" ]; then
+		if f_isset _${var}_delete; then
 			unset _${var}_delete
 		else
 			setvar _${var}_delete 1