umdns: update to the version 2017-03-21

This includes following changes:
480d7bc Fix sending unicast questions on cache expire
a0403cd Keep source sockaddr for every cached DNS record
1478293 Fix code freeing cached non-A(AAA) records too early
9f1cc22 Fix replying to "QU" questions received on unicast interface
943bedb Fix reading port of incoming packets
c725494 Use MCAST_PORT define for port 5353
ce7e9e9 Use one define for DNS-Based Service Discovery service name
e1bacef Drop entries cached for interface we're going to delete
496aeba Fix comment typo in cache_gc_timer
f89986b Fix refreshing cached A(AAA) records that expire

Previous updates made umdns work as expected on startup but there were
still many bugs. They were mostly related to runtime - cache management
and requests + responses. E.g. umdns was never able to send question on
DNS record expire. It was also ignoring all incoming unicast questions.

Since these issues are quite serious it makes sense to backport this
update to the stable branch.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>