[CLEANUP] move remaining stats sockets code to dumpstats

[CLEANUP] move remaining stats sockets code to dumpstats

The remains of the stats socket code has nothing to do in proto_uxst
anymore and must move to dumpstats. The code is much cleaner and more
structured. It was also an opportunity to rename AN_REQ_UNIX_STATS
as AN_REQ_STATS_SOCK as the stats socket is no longer unix-specific
either.

The last item refering to stats in proto_uxst is the setting of the
task's nice value which should in fact come from the listener.