cmd/hotfix: add hotfix tool

[zyga]

The hotfix tool is intended to help people that are tracking edge and
get into funny state that is hard to recover manually (e.g. by
refreshing to another core revision) but can still be fixed
automatically.

Along with the simple tool we also add the first hotfix, the one that is
affecting people on the core revision 1968, where core refresh fails
with "snap-update-ns" errors.

Signed-off-by: Zygmunt Krynicki zygmunt.krynicki@canonical.com

[codecov-io]

Codecov Report

Merging #3410 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3410      +/-   ##
==========================================
+ Coverage   77.52%   77.54%   +0.01%     
==========================================
  Files         369      369              
  Lines       25475    25475              
==========================================
+ Hits        19750    19755       +5     
+ Misses       3974     3970       -4     
+ Partials     1751     1750       -1

Impacted Files	Coverage Δ
overlord/snapstate/snapstate.go	81.89% <0%> (+0.24%)	⬆️
overlord/ifacestate/helpers.go	66.38% <0%> (+0.84%)	⬆️
interfaces/sorting.go	96.66% <0%> (+3.33%)	⬆️

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[niemeyer]

This is exactly what the repair tool is supposed to handle. Let's please take care of it there.