netlink: return of setDaemon()
docs: start to document the messaging system
docs: messaging.rst update
iocore: separate mgmtmsg and ctrlmsg classes

Separate namespaces of the generic Netlink control messages' NLAs and
pyroute2 messaging system control messages' NLAs.

mgmtmsg class is still in generic.py, should be extracted into a separate
module later.
iocore: cumulative commit, custom services

* custom services register
* service discovery
* proper addr usage
* fix envelope stream parsing

FIXME: deprecates parse_envelope?