resolvconf: /etc/resolv.conf file format parser

[[unix::resolvconf]] implements a parser for /etc/resolv.conf files which has
feature parity with the resolv.conf format supported by glibc 2.36. However,
most options are not supported by Hare internally, i.e. via [[net::dns]].

The user may parse a resolv.conf file manually via the [[read]] and [[next]]
functions. Additionally, this module maintains a global copy of the local
resolv.conf file, parsed once at runtime and cached for the lifetime of the
process, which is available via [[load]].
