nl_read may start new peer that we need to add in the pollfd pool.

When looking for the interface index of a configured address, check if
the interface address returned by getifaddrs is not null.
As described by the man page, ifa_addr can be a null pointer.

Only set retx passes (and print the count) when retx will actually happen

See merge request !1

This option lets the user set a delay in 27Mhz ticks at startup before
going up if no message is received.
This can be useful when joining the multicast group takes time.

The poll timeout wasn't updated when triggered too soon, it's fixed.
And 1ms is added to the timeout to compensate precision loss.

Depending on the skew value and announce periodicity, the master
expiration date might be reset all the time. In this case multilive will
never get up.

struct ip is available on Apple systems.

Fix this warning from glibc:

/usr/include/features.h:183:3: warning: #warning "_BSD_SOURCE and
_SVID_SOURCE are deprecated, use _DEFAULT_SOURCE" [-Wcpp]

Keep _BSD_SOURCE for backward compatibility.