Change w(1) to compute FROM (host) field size dynamically

It's nice to be able to display a full IPv6 host address if
needed, but it's also nice to display more than 3 characters of a command
line. Compute the needed size for the FROM column in an earlier pass,
and determine the maximum, then print what fits for the command.

Reviewed by: marcel@ (markm@ previous revision)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D21211