vrrp: fix double close issue (DROP_MEMBERSHIP & netlink channel)

This is a old pending 'bug', not arming at all but just frustrating
to see again and again this log message :

"cant do IP_DROP_MEMBERSHIP errno=Bad file descriptor (9)"

What the hell ! it was due to a double close during reload &
stop procedure. VRRP fd are stored in a socket pool and use
the I/O MUX to handle VRRP traffic. While reloading or stopping
the daemon the I/O MUX was released first and secondly socket pool.
The issue spotted here, in thread_destroy_master() all pending
thread are canceled and read/write fds related are close().

Well OK a close on a mcast socket perform kernel side the
DROP_MEMBERSHIP when needed, but it is much more clean to
perform proper operations userspace !

This patch sequencely cancel pending thread, release socket pool
and finally destroy master thread.

Same 'issue' appear in netlink channel.