First round of netstat(1) cleanup.

First round of netstat(1) cleanup.

Removed the ambiguity in -s, -f, -p and -i flags handling.
Basically, there are four displays (except others):

1.  PCB display.
2.  Protocol statistics display. (-s)
3.  Interface statistics display. (-i)
4.  Per-interface protocol statistics display. (-i -s)

All of the above except 3) can be limited to a particular
protocol family (-f) or a single protocol (-p).

Some examples:

1.  netstat -f inet    -- show PCBs of all INET protocols
2.  netstat -p udp     -- show PCB of UDP protocol only (NEW!)
3.  netstat -s         -- show protocol statistics for all families
4.  netstat -s -f inet -- show INET protocols statistics
5.  netstat -s -p icmp -- show ICMP protocol statistics

This is a work in progress.  Manpage has been fixed slightly,
but is still incomplete.