BUGFIX: Argument order `pw usermod` command.

The argument order of `pw usermod` is[1]:

    pw [-V etcdir] usermod [name|uid]

This was incorrect for the `chhome` function. The updated version is
tested on FreeBSD.

[1]: http://www.freebsd.org/cgi/man.cgi?query=pw&sektion=8
Merge pull request #8794 from wunki/pw_user-chhome

BUGFIX: Argument order `pw usermod` command.