mfc-candidates: Convert to Lua

d51c59002367 added a Lua script to process the lists of candidate and
completed MFC commits to address sorting issues in the original shell
implementation.

Instead of having a mix of shell and Lua, just implement the entire
tool in Lua. This is more maintainable and gives a reasonable
improvement in performace.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47416