Add basic NetBSD network support:

- use "ifconfig -a", because older NetBSD doesn't support that as a default,
  while continuing to work on FreeBSD and Darwin.

- keyword for MAC address is "address:", not "ether"

- ipv4 secondary addresses have an "alias" keyword, which needs to be
  removed.

Tested on NetBSD 5.1, NetBSD 6.0, FreeBSD 9.0 and MacOS 10.8.2.