Improve relative>absolute filename option handling

Relative -> absolute filename handling now occurs for the following flags:
 -I
 -F
 -iquote
 -include

Rather than just -I.

Backslashes are now no longer erroneously removed, and can be used as an
escape character on posix systems.

Relative->absolute filename handling should be more robust in general.
Merge pull request #390 from Andrew-Dunn/better_absolute_paths

Improve relative>absolute filename option handling