now that adduser allows us to set its home directory, we can do
this using adduser instead of using usermod. this change also
silences the warning from lintian
"maintainer-script-lacks-home-in-adduser". lintian complains if
`adduser --system` is called without passing `--home` option.
also, take this opportunity to s/-c/--comment/ in the command line
of `usermod`, for better readability.
Fixes: https://tracker.ceph.com/issues/64069
Signed-off-by: Kefu Chai <tchaikov@gmail.com>
