add flux module remove --cancel option

[garlick]

This adds an option to remove an ornery module by sending it a pthread_cancel(3) instead of politely requesting that it shut down. That required the addition of a pthread cleanup callback, which triggered a bit of refactoring.

[garlick]

Thanks!

[codecov[bot]]

Codecov Report

:x: Patch coverage is 88.88889% with 4 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 83.85%. Comparing base (4e80d20) to head (1221851). :warning: Report is 621 commits behind head on master.

Files with missing lines	Patch %	Lines
src/broker/modhash.c	77.77%	2 Missing :warning:
src/broker/module.c	92.00%	2 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6894      +/-   ##
==========================================
- Coverage   83.85%   83.85%   -0.01%     
==========================================
  Files         538      538              
  Lines       90029    90041      +12     
==========================================
+ Hits        75491    75501      +10     
- Misses      14538    14540       +2     

Files with missing lines	Coverage Δ
src/cmd/flux-module.c	80.00% <100.00%> (+0.04%)	:arrow_up:
src/broker/modhash.c	80.41% <77.77%> (+0.20%)	:arrow_up:
src/broker/module.c	81.40% <92.00%> (+1.81%)	:arrow_up:

... and 8 files with indirect coverage changes

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