refactor: Make tox-bootstrapd use bool instead of int

[nurupo]

A continuation of the cleanup done in #2621.

tox-bootrstrapd historically had used ints for boolean values, as it was initially written in C89 which has no stdbool.h. Since then it has modernized and moved on to using C11, but the usage of the int type to represent boolean values, "boolean ints", remained. Recently, driven by a desire to eliminate implicit int-to-bool conversion, @iphydf did a cleanup in b7404f2, changing some of the boolean ints to bools and doing manual int-to-bool conversion on the remaining boolean ints. This left the codebase in an inconsistent state of both ints and bools now being used to represent boolean values, not to mention that the explicit int-to-bool conversions are a bit ugly. The only boolean ints that remained are those stemming from libconfig's config_lookup_bool() taking an *int parameter to return a boolean value, as libconfig still uses C89. This commit adds a wrapper function around libconfig's config_lookup_bool() that takes a *bool instead, eliminating the remaining boolean ints and majority of the explicit int-to-bool conversions in tox-bootstrapd.

---

This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.09%. Comparing base (fa20168) to head (395e048).
Report is 11 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2692      +/-   ##
==========================================
+ Coverage   73.03%   73.09%   +0.06%     
==========================================
  Files         149      149              
  Lines       30531    30531              
==========================================
+ Hits        22298    22318      +20     
+ Misses       8233     8213      -20     

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

[iphydf]

Reviewed 4 of 4 files at r1.
Reviewable status: complete! 1 of 1 approvals obtained