Improve defaults and environment management

[gilesknap]

update config.py to supply all the defaults and
environment variable overrides

update the Unit description for the client service so that it overrides the defaults for client socket and configuration file location.

[codecov]

Codecov Report

❌ Patch coverage is 81.57895% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.96%. Comparing base (1d4f683) to head (312922c).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/usb_remote/utility.py	20.00%	4 Missing ⚠️
src/usb_remote/client_api.py	50.00%	2 Missing ⚠️
src/usb_remote/server.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #33      +/-   ##
==========================================
+ Coverage   81.51%   81.96%   +0.44%     
==========================================
  Files          12       12              
  Lines         952      948       -4     
==========================================
+ Hits          776      777       +1     
+ Misses        176      171       -5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.