BUG/MEDIUM: systemd: set KillMode to 'mixed'

By default systemd will send SIGTERM to all processes in the service's
control group. In our case, this includes the wrapper, the master
process and all worker processes.

Since commit c54bdd2a the wrapper actually catches SIGTERM and survives
to see the master process getting killed by systemd and regard this as
an error, placing the unit in a failed state during "systemctl stop".

Since the wrapper now handles SIGTERM by itself, we switch the kill mode
to 'mixed', which means that systemd will deliver the initial SIGTERM to
the wrapper only, and if the actual haproxy processes don't exit after a
given amount of time (default: 90s), a SIGKILL is sent to all remaining
processes in the control group. See systemd.kill(5) for more
information.

This should also be backported to 1.5.
(cherry picked from commit 74f016985ab1fa7a6a5afa301d982e77eba9c96a)