ENH: Config override spec sanity checks (fixes gh-3451)

[mih]

To prevent us from shooting ourselves in the foot. Without
this change, we could have specified section-less variables
that our own ConfigManager cannot handle, and don't seem to
be supported by Git either.

[codecov]

Codecov Report

Merging #3695 into master will decrease coverage by 0.03%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #3695      +/-   ##
==========================================
- Coverage   82.88%   82.85%   -0.04%     
==========================================
  Files         273      274       +1     
  Lines       35797    35826      +29     
==========================================
+ Hits        29670    29683      +13     
- Misses       6127     6143      +16

Impacted Files	Coverage Δ
datalad/cmdline/main.py	77.73% <66.66%> (-0.29%)	⬇️
datalad/auto.py	86.12% <0%> (-1.82%)	⬇️
datalad/downloaders/tests/test_http.py	60% <0%> (-1.24%)	⬇️
datalad/support/lzma.py	60% <0%> (ø)
datalad/tests/test_auto.py	88.62% <0%> (+0.2%)	⬆️

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[mih]

Second appveyor run times out -- unlikely to be related to this change.