iftop zone support

Unlike many other Unix variants, Solaris draws a clear separation between
an IP interface and a datalink.

iftop will support the IP and datalink interfaces in global zones, and
support only the IP interface in exclusive zones and with vanity naming.
It will use libpcap, socket API, and ioctls with the IP interface. The
DLPI interface included with iftop will be removed.

All changes will be sent to the upstream community, and the man pages
will be updated accordingly.