Use EXIT_SUCCESS and EXIT_FAILURE for exit()

[znewt99]

Use the EXIT_SUCCESS and EXIT_FAILURE constants from stdlib

[codecov]

Codecov Report

❌ Patch coverage is 0% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.90%. Comparing base (61f5ce7) to head (29d9e6a).

Files with missing lines	Patch %	Lines
md2html/md2html.c	0.00%	10 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #403   +/-   ##
=======================================
  Coverage   78.90%   78.90%           
=======================================
  Files           5        5           
  Lines        3859     3859           
  Branches     1295     1295           
=======================================
  Hits         3045     3045           
  Misses        299      299           
  Partials      515      515           

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