Add a man page for Linux installs
Follow-up of #13 (closed). The man page should be installed as ${CMAKE_INSTALL_PREFIX}/share/man/man6/lugaru.6
(it then gets compressed automatically by most distro package managers).
Since ff29f47f we now have a proper help output. It could be parsed with a tool such as help2man
to generate a man page automatically.
Alternatively we can also maintain a manpage manually if we don't intend to modify command line arguments too often.