user module: add support for specifying skel_dir

All supported plattforms of the user module can take a `-k` option
to specify a different skeleton directory than the system default
(/etc/skel).

Man page references:

Linux: http://man7.org/linux/man-pages/man8/useradd.8.html
FreeBSD: http://www.freebsd.org/cgi/man.cgi?query=pw&section=8
OpenBSD: http://www.openbsd.org/cgi-bin/man.cgi?query=useradd&sektion=8
NetBSD: http://netbsd.gw.com/cgi-bin/man-cgi?useradd++NetBSD-current
Solaris: http://illumos.org/man/useradd
AIX: http://pic.dhe.ibm.com/infocenter/aix/v7r1/index.jsp?topic=/com.ibm.aix.cmds/doc/aixcmds5/useradd.htm