reflectd: better handling for RSTs from the destination

A FIN / RST packet from the destination host (ie. the one that traffic
is being mirrored to) may be received after epoll_wait returns, so we
need to handle additional errors when calling write.

This requires two changes:

1. Ignore SIGPIPE so that it can't kill the entire process due to a
   single closed socket.

2. Remove a redundant "connection closed by peer" log message that was
   caused by the same race.