FIX: issue #277 (Include system doesn't handle well files with same name in different directories)

This is a deep change in the Red/System loader, the include-dirs sub-system has been replaced by a stack-based approach. This should fix more issue than just this one. In order to make sure this fix doesn't break anything and, maybe, to push it further to fix more includes issues, it has been pushed in its own branch.
FIX: attempt at fixing issues #251 and #252.
FIX: new fix for issues #251 and #252.
FIX: regression on generating an error when the input script file is missing.
FIX: deeper fix for issues #251 and #252.
FIX: missing change for previous commit.
FIX: issue #366 (Compilation of code outside of the Red/ path fails under Windows)
FIX: extended last fix for #366.
FIX: use system/script/parent/path to tackle #366
Merging `fix-issue-277` branch.

Fixes following issues:
#251 (Red doesn't find source in working directory)
#252 (#system cannot find #include)
#277 (Include system doesn't handle well files with same name in different directories)
#366 (Compilation of code outside of the Red/ path fails under Windows)
#381 (#system-global doesn't detect equal #include paths)