systemd: Use non-absolute paths in Exec* lines
Since systemd v239, Exec* binaries are resolved from PATH when they
are not-absolute. Switch to this by default for ease of downstream
maintenance. Many downstream distributions move individual binaries
to locations that existing compile-time configurations cannot
accommodate.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Dimitri John Ledkov <dimitri.ledkov@canonical.com>
Closes #14880