services_captiveportal_zones_edit.php: Zone name cannot start with a number
The zone name on services_captiveportal_zones_edit.php is used as an XML tag name and needs to follow the relevant standards so it does not produce invalid XML.
Currently the page allows starting the zone name with a number, which results in an invalid XML tag and the config change is reverted.
Fixed #6514 by requiring string starts with letter ot underscore
(cherry picked from commit f0a053846d6cde2724c47b5553e1395cfd21445c)