Rename UserConfig and LegacyChannelConfig fields

[wpaulino]

The current names aren't very clear to what each field represents, this commit aims to improve that.

Follow-up from #1529.

[codecov-commenter]

Codecov Report

Merging #1540 (9f18026) into main (4356809) will increase coverage by 0.02%.
The diff coverage is 92.46%.

❗ Current head 9f18026 differs from pull request most recent head 44fa3ac. Consider uploading reports for the commit 44fa3ac to get more accurate results

@@            Coverage Diff             @@
##             main    #1540      +/-   ##
==========================================
+ Coverage   90.94%   90.96%   +0.02%     
==========================================
  Files          80       80              
  Lines       43533    43533              
  Branches    43533    43533              
==========================================
+ Hits        39589    39599      +10     
+ Misses       3944     3934      -10     

Impacted Files	Coverage Δ
lightning/src/util/config.rs	61.62% <69.23%> (ø)
lightning/src/ln/channel.rs	88.58% <85.71%> (ø)
lightning-invoice/src/utils.rs	96.63% <100.00%> (ø)
lightning/src/ln/chanmon_update_fail_tests.rs	97.71% <100.00%> (ø)
lightning/src/ln/channelmanager.rs	84.34% <100.00%> (ø)
lightning/src/ln/functional_test_utils.rs	95.23% <100.00%> (ø)
lightning/src/ln/functional_tests.rs	97.16% <100.00%> (+0.10%)	⬆️
lightning/src/ln/onion_route_tests.rs	97.51% <100.00%> (ø)
lightning/src/ln/payment_tests.rs	99.24% <100.00%> (ø)
lightning/src/ln/priv_short_conf_tests.rs	96.60% <100.00%> (ø)
... and 4 more

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[tnull]

LGTM, thank you very much!