ifconfig(8): Add '-f' option to print in more formats

ifconfig(8): Add '-f' option to print in more formats

* Add the '-f' option to control the output formats of addresses,
  inet (IPv4) subnet masks, inet6 (IPv6) prefix length, Ethernet (MAC)
  address.  The following output formats are supported:

  - address: numeric (default), host, FQDN
  - inet (IPv4) subnet masks: CIDR, dotted-quad, and traditional hex
    output (default)
  - inet6 (IPv6) prefix length: CIDR, traditional integer format
    (default)
  - ether (MAC address): colon-separated (default), dash-separated

* The 'IFCONFIG_FORMAT' environment variable has the same usage as the
  '-f' option and can be overrided by the latter.

* Some minor cleanups.

Obtained from FreeBSD (revisions 301059 and 301185; review D2856), but
with my own minor modifications.