Take reyk's imsg resolv.conf improvements of a while ago to their

Take reyk's imsg resolv.conf improvements of a while ago to their
logical conclusion.

Nuke _PATH_RESOLV_CONF since the value is only meant to be
known inside priv_write_resolv_conf(). Just use a local const char *.

Bring priv_write_resolv_conf() into line with other priv_ functions
invoked from the dispatch loop. i.e. don't pass it the imsg, just pass
a pointer to the data and a size after ensuring there is data to pass.