Prevent duplicate renderd sections in renderd.conf

[hummeltech]

In order to remove any confusion if the user has duplicate renderd sections in their renderd.conf file.

Currently with a configuration like this one:

[renderd]
pid_file=/run/renderd/renderd.pid
socketname=/run/renderd/renderd.sock
stats_file=/run/renderd/renderd.stats
tile_dir=/var/cache/renderd/tiles

[renderd0]
pid_file=/run/renderd/renderd0.pid
socketname=/run/renderd/renderd0.sock
stats_file=/run/renderd/renderd0.stats
tile_dir=/var/cache/renderd/tiles

only the second renderd section (renderd0) will actually be used.

After this patch is merged, an error message will be shown and the application will exit.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.41%. Comparing base (aebb506) to head (13e134c).
⚠️ Report is 51 commits behind head on master.
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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #421   +/-   ##
=======================================
  Coverage   80.40%   80.41%           
=======================================
  Files          24       24           
  Lines        5930     5932    +2     
=======================================
+ Hits         4768     4770    +2     
  Misses       1162     1162           

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