Fix SIGTERM behavior that causes systemd control of etserver to timeout.

[jshort]

From various user reports, systemctl stop et hangs and times out
requiring a SIGKILL signal. In addition to adding the hook to kill
sentry/telemetry on SIGTERM, we need to exit as well.

Fixes #546

[jshort]

 ❯ ps -ef | grep etserver                                                                                                                                                                                            09:43:43  12.16.22
root     1161316       1  0 Dec15 ?        00:02:20 /usr/bin/etserver --cfgfile=/etc/et.cfg
 ✔  ⚙  jwshort@host  ~/git/EternalTerminal/build   etserver_sigterm_fix 
 ❯ sudo systemctl status et                                                                                                                                                                                          09:43:48  12.16.22
● et.service - Eternal Terminal
     Loaded: loaded (/usr/lib/systemd/system/et.service; enabled; preset: disabled)
    Drop-In: /etc/systemd/system/et.service.d
             └─execwall.conf, restart-on-failure.conf
     Active: active (running) since Thu 2022-12-15 16:24:08 PST; 17h ago
   Main PID: 1161316 (etserver)
         IO: 56.0K read, 116.0K written
      Tasks: 11
     Memory: 1.3M
        CPU: 2min 20.417s
     CGroup: /system.slice/et.service
             └─1161316 /usr/bin/etserver --cfgfile=/etc/et.cfg

Notice: journal has been rotated since unit was started, output may be incomplete.
 ✔  ⚙  jwshort@host  ~/git/EternalTerminal/build   etserver_sigterm_fix 
 ❯ sudo systemctl stop et                                                                                                                                                                                            09:43:59  12.16.22
 ✔  ⚙  jwshort@host  ~/git/EternalTerminal/build   etserver_sigterm_fix 
 ❯ ps -ef | grep etserver                                                                                                                                                                                            09:44:08  12.16.22
 ✔  ⚙  jwshort@host  ~/git/EternalTerminal/build   etserver_sigterm_fix 
 ❯ sudo systemctl status et                                                                                                                                                                                          09:44:09  12.16.22
× et.service - Eternal Terminal
     Loaded: loaded (/usr/lib/systemd/system/et.service; enabled; preset: disabled)
    Drop-In: /etc/systemd/system/et.service.d
             └─execwall.conf, restart-on-failure.conf
     Active: failed (Result: exit-code) since Fri 2022-12-16 09:44:08 PST; 5s ago
   Duration: 17h 19min 58.850s
    Process: 1161316 ExecStart=/usr/bin/etserver --cfgfile=/etc/et.cfg (code=exited, status=15)
   Main PID: 1161316 (code=exited, status=15)
         IO: 56.0K read, 116.0K written
        CPU: 2min 20.441s

Dec 16 09:44:07 host systemd[1]: Stopping et.service - Eternal Terminal...
Dec 16 09:44:08 host etserver[1161316]: Got interrupt (perhaps ctrl+c?).  Exiting.
Dec 16 09:44:08 host systemd[1]: et.service: Main process exited, code=exited, status=15/n/a
Dec 16 09:44:08 host systemd[1]: et.service: Failed with result 'exit-code'.
Dec 16 09:44:08 host systemd[1]: Stopped et.service - Eternal Terminal.
Dec 16 09:44:08 host systemd[1]: et.service: Consumed 2min 20.441s CPU time, read 56.0K from disk, written 116.0K to disk.
Notice: journal has been rotated since unit was started, output may be incomplete.

[MisterTea]

Nice fix!