debugger: cleanup code

Get rid of all irrelevant for the Tarantool code:
- branches of code for non-LuaJIT interpreter;
- LuaJIT, but Windows-specific code;
- luadebug.lua (former debugger.lua) readline functions
  were incompatible with Tarantool console readline,
  so disable it entirely;
- Also fixed multiple luacheck warnings.

NO_DOC=refactoring
NO_CHANGELOG=refactoring
NO_TEST=refactoring