We should check type output to ensure that we're really calling a shell function and not some badly named binary like stop from boinc, Gentoo #218063.

sh/functions.sh.in

@@ -20,6 +20,15 @@ eoutdent()
 	return 0
 }
 
+is_function()
+{
+	if [ -n "${BASH_VERSION}" ]; then
+		[ "$(type -t "$1")" = "function" ]
+	else
+		[ "$(type "$1" 2>/dev/null)" = "$1 is a shell function" ]
+	fi
+}
+
 yesno()
 {
 	[ -z "$1" ] && return 1