BUGFIX: Argument order `pw usermod` command.

The argument order of `pw usermod` is (see [man]):

    pw [-V etcdir] usermod [name|uid]

This was incorrect for the `chhome` function. The updated version is
tested on FreeBSD.

[man]: http://www.freebsd.org/cgi/man.cgi?query=pw&sektion=8