rpm: avoid hiding errors from systemd commands

Humans are susceptible to making orthographic errors sometimes. A
misspelled "%systemd_post caek.service" would go unnoticed due to all
output from systemctl being discarded if and when %post runs.

To alleviate this, cease hiding outputs. Then, to account for the
potential absence of systemd from the system, add file checks so as
to not generate a "command not found" error.