rpm: fix %systemd_user_post() macro.

Escape "--user" and "--global" arguments with "\\" since rpm treats
arguments starting with "-" as macro options which causes "Unknown
option" rpm error.
Use %{expand:...} to force expansion of the inner macro. Otherwise %{?*}
is recursively defined as "\--user \--global {%?*}" which causes
"Too many levels of recursion in macro expansion" rpm error.

Thanks to Michael Mráka for helping me fix the above issues.