Add an option for starting a gops agent

[arnested]

Maybe a gops agent can help us find out why ldddns sometimes locks up.

This should be considered an “undocumented” feature, and it might be removed again anytime.

The agent won't be started by default but needs to be explicitly enabled by setting LDDDNS_GOPS=true.

We also need to loosen systemd's tight grip on the process a bit. Add a /etc/systemd/system/ldddns.service.d/gops.conf file with the following content:

[Service]
Environment=LDDDNS_GOPS=true
PrivateNetwork=no
ProtectHome=no
RestrictAddressFamilies=AF_UNIX AF_INET
IPAddressAllow=127.0.0.1

[codecov]

Codecov Report

❌ Patch coverage is 0% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 29.81%. Comparing base (819abb2) to head (4fce1ce).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
main.go	0.00%	11 Missing ⚠️
entry_groups.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #440      +/-   ##
==========================================
- Coverage   30.81%   29.81%   -1.01%     
==========================================
  Files           7        7              
  Lines         357      369      +12     
==========================================
  Hits          110      110              
- Misses        245      257      +12     
  Partials        2        2              

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