Add a valid_user type

This types is meant to be used to check if strings provided are
valid user names. This is however designed to be overriden by
site-specific policies. The regex used as default for this type
is the one suggested by the useradd man page.

quattor/functions/validation.pan

@@ -139,3 +139,11 @@ function valid_absolute_file_paths = {
     };
     true;
 };
+
+@documentation{
+    desc = This types is meant to be used to check if strings provided are \
+        valid user names. This is however designed to be overriden by \
+	site-specific policies. The regex used here as default is the \
+	one suggested by the useradd man page.
+}
+type valid_user = string with match(SELF, "^[a-zA-Z_][a-zA-Z0-9_-]{0,30}([a-zA-Z0-9_-]|\$)?$");