debian/ceph-common.postinst: set user directory using adduser

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>

debian/ceph-common.postinst

@@ -52,16 +52,20 @@ case "$1" in
                  --system \
                  --no-create-home \
                  --disabled-password \
+                 --home $SERVER_HOME \
                  --uid $SERVER_UID \
                  --gid $SERVER_GID \
                  $SERVER_USER 2>/dev/null || true
          echo "..done"
        fi
        # 3. adjust passwd entry
+       # NOTE: we should use "adduser --comment" if we don't need to
+       # support adduser <3.136. "adduser --gecos" is deprecated,
+       # and will be removed, so we don't use it. the first distro
+       # using --comment is debian/trixie or ubuntu/mantic.
        echo -n "Setting system user $SERVER_USER properties.."
-       usermod -c "$SERVER_NAME" \
-               -d $SERVER_HOME   \
-               -g $SERVER_GROUP  \
+       usermod --comment "$SERVER_NAME" \
+               --gid $SERVER_GROUP      \
                $SERVER_USER
        # Unlock $SERVER_USER in case it is locked from an uninstall
        if [ -f /etc/shadow ]; then