Record ucast/bcast/mcast flags in a packet's buffer when received...

...rather than computing them in ntp_monitor.c, in order to avoid having
to keep data (the socket fd) that cannot be serialized into a replay file.

The SOCKET fd field is now used only for purging fd-associated recvbufs (which
will be bypassed in replay mode) and for diagnostic messages on the refclock
side of things.