config: remove 'ignore' config option

config: remove 'ignore' config option

1. Remove 'ignore' config option for interface. Interface is still used if
   any of 'ra', 'ndp', 'dhcpv4' or 'dhcpv6' options are different from
   default 'disabled' mode. In config_parse_interface() update runtime
   ignore value according to all mode values.
2. Reset ignore, ra, ndp, dhcpv4 and dhcpv6 values in set_interface_defaults().
   Otherwise if option is deleted from config file and config is reloaded,
   it will use the value from old config, potentiallly not turning services off.
3. Do not use implicit checks that 0 server mode means MODE_DISABLED.
4. Simplify code of all service setup functions if passed enabled = true
   argument, but service mode is disabled.

Signed-off-by: Dainis Jonitis <dainis.jonitis@ubnt.com>
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>