Implement dnsd

Created by: batonius

Currently, there's no centralized DNS subsystem in Redox and two independent implementations of lookup_host in stdlib and newlib. By implementing a specialized daemon we'll be able to cache DNS requests, perform reverse queries, and provide clear, user-friendly interface to DNS queries.

The new daemon should implement dns: schema, so a process could open dns:example.org and read resolved IPv4 (for now) in string format. Other record types could be implemented in the future as "dns:aaaa/example.org". The daemon should keep resolved domains in a cache for a configurable amount of time.

Once #7 (closed) is implemented, the daemon should get the addrs of the DNS server from netcfg:resolv/server.