Revert "systemd: Use non-absolute paths in Exec* lines"

This reverts commit 79b20949b25c8db4d379f6486b0835a6613b480c since it
doesn't work with the systemd version shipped with RHEL7-based systems.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #14943
Closes #14945