rpm: check argument counts for systemd macros

Invoking %systemd_tmpfiles (in %post) without any arguments, while
possible, will cause systemd-tmpfiles to process the entire system
configuration, rather than just the newly installed configuration
files. In https://github.com/systemd/systemd/pull/12048, it was
established that processing everything constitutes unusual practice,
and should be flagged as a mistake at build time.

Furthermore, invoking %systemd_post without any arguments will cause
the underlying `systemctl preset` to outright return an error ("Too
few arguments") when run. This can be flagged during build time in
the same manner.

As I have found no ways to successfully nest %if clauses inside a
macro[1], I am helping myself by reusing the recursive variable
expansion technique pioneered in [2].

Now, when %systemd_post or %systemd_tmpfiles is incorrectly used,
rpm gives accurate line number reporting, too:

	error: This macro requires some arguments
	error: line 11: %{systemd_post}

	error: This macro requires two arguments
	error: line 13: %{tmpfiles_create_package meh more more}

[1] what has been tried: %{expand:%%if "%#" == 0 \\\
    %%{error:you have given me %# args} \\\
    %%endif}

[2] http://git.savannah.gnu.org/cgit/automake.git/commit/?id=e0bd4af16da88e4c2c61bde42675660eff7dff51