Add support for systemd watchdog timer

[csstaub]

This adds support for systemd watchdog timer. Still need to add more tests.

[codecov]

Codecov Report

Attention: Patch coverage is 97.26027% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 79.12%. Comparing base (3a8fd6e) to head (d4c6720).
Report is 2 commits behind head on master.

Files	Patch %	Lines
status.go	96.66%	0 Missing and 1 partial ⚠️
status_linux.go	97.05%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #427      +/-   ##
==========================================
+ Coverage   78.40%   79.12%   +0.72%     
==========================================
  Files          31       33       +2     
  Lines        2135     2204      +69     
==========================================
+ Hits         1674     1744      +70     
+ Misses        381      380       -1     
  Partials       80       80              

Flag	Coverage Δ
darwin	77.91% <97.43%> (+0.45%)	⬆️
linux	84.13% <97.01%> (+0.64%)	⬆️

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[csstaub]

Template for a systemd service file (requires systemd >=253):

[Unit]
Description=Ghostunnel
After=network.target

[Service]
Type=notify-reload
ExecStart=...
ReloadSignal=SIGUSR1
WatchdogSec=1
Restart=always

[Install]
WantedBy=default.target