The GNU/Hurd platform does not define IP_ADD_SOURCE_MEMBERSHIP,
IP_DROP_SOURCE_MEMBERSHIP, MCAST_JOIN_SOURCE_GROUP and
MCAST_LEAVE_SOURCE_GROUP.
Implement a few functions for the GNU/Hurd. Specifically:
* uv_resident_set_memory (from Linux)
* uv_get_free_memory (from Linux)
* uv_get_total_memory (from Linux)
* uv_cpu_info (from cygwin)
* uv__process_title_cleanup (void)
* uv_get_constrained_memory (stub)
* Leave proctitle unimplemented on Hurd for now
* Implement hurdish uv_exepath
* Enable ifaddrs api
* Unbreak udp basics
* Unbreak futime and lutime on Hurd