Introduce a new helper function check_search() derived for res_hnok to
check the domain-name parameter according to the rules for "search"
strings as documented in resolv.conf(5). Specifically, the string must
be no more than 256 bytes long and contain no more than six valid domain
names separated by white space.

The previous unchecked values could result in a mangled resolv.conf
file which could effectively deny access to local sites. This is not
a security issue as rogue dhcp servers could already do this without
sending invalid strings.

Reviewed by: cperciva
MFC After: 3 days