Fix systemd spec file macros

Fix systemd spec file macros

Ensure that the _unitdir, _presetdir, _modulesloaddir, and
_systemdgeneratordir macros are always defined.  If not set
them to the expected default values.  Pass all of these options
to ./configure and package the resulting files in those locations.

Additionally, set __brp_mangle_shebangs_exclude_from until the
conversion to Python 3 is complete so they may be built cleanly
under mock.

Reviewed-by: Neal Gompa <ngompa@datto.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7567
Closes #8119