[sslh] Extend sslh.service

[Thomas Weißschuh]

Hi,

it may be useful *add* following directives to their respective sections
in the sslh.service systemd unitfile:

[Unit]
After=network.target

As all network services are supposed to start after network.target sslh
isn't really useful before.

The following is only useful for sslh-fork but shouldn't harm
sslh-select.

[Service]
KillMode=process

Explained in systemd.kill(5)
This makes `systemctl restart` kill *only* the main process.
The forked children and therefore their associated connections are kept alive.
This makes upgrades *not* break running connections.

If killing all processes is desired, e.g. for security upgrades, one can
use `systemctl kill` prior to `systemctl restart` to terminate all
processes.

As a sidenote:
One can do `systemctl kill --kill-who=main` to kill the listening
process and therefore prevent new connections while preserving already
running connections.

Thank you for sslh and regards,
Thomas