Do not ignore resolv.conf syntax errors

Do not ignore resolv.conf syntax errors

irs_resconf_load() stores the value returned by add_search() into ret
without consulting its current value first.  This causes any previous
errors raised while parsing resolv.conf to be ignored as long as any
"domain" or "search" statement is present in the file.

Prevent this by returning early in case an error is detected while
parsing resolv.conf.  Ensure that "searchlist" and "magic" members of
the created irs_resconf_t structure are always initialized before
isc_resconf_destroy() is called.