odhcpd: rename iface->search -> iface->dns_search

odhcpd: rename iface->search -> iface->dns_search

And fix some bugs/nits in the process, e.g.:
 - realloc() not handled properly in config.c
 - replace a number of magic values
 - in router.c, the DNSSL options has 8 bytes of superfluous padding

Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/320
Signed-off-by: Álvaro Fernández Rojas <[email protected]>