luci-mod-network: rework DHCP relay settings

 - Drop completely bogus 'ID' option, it's not handled by anything
 - Clarify relay from / relay to / accept from options

Fixes: #6639
Fixes: a627744b

 ("luci-mod-network: 'relay' tab added")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>