netstat -i: do not truncate interface names

The field for interface names for netstat -i was 5 characters by
default, which is no longer sufficient with names like "vlan1234"
and "vtnet0". netstat -iW computed the necessary field width, but
also enlarged the address field by a lot (especially with IPv6 enabled).
Make netstat -i compute the field width for interface names with or
without -W. Note that the existing default output does not fit in
80 columns in any case. Update the man page accordingly, documenting
the remaining effect of -W with -i. Also add -W to the list of
General Options, as there are numerous pointers to this.

Reported by: Chris Ross
Reviewed by: melifaro, rgrimes, cy
Differential Revision: https://reviews.freebsd.org/D35703
MFC after: 1 week