Fix systemd_ RPM macros usage on Debian-based distributions

Debian-based distributions do not seem to provide RPM macros for
dealing with systemd pre- and post- (un)install actions: this results
in errors when installing or upgrading .deb packages because the
resulting control scripts contain the following unresolved macros:

• %systemd_post
• %systemd_preun
• %systemd_postun

Fix this by providing default values for postinstall, preuninstall and
postuninstall scripts when these macros are not defined.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #7074
Closes #7100