Correctly find main file when user has non-standard magicness

Adds a \m to the search() call in GetMainTexFile(), which otherwise breaks if the user sets 'nomagic' or some other non-default magicness.
This would break LaTeX-Box realising that the current file is a main file in those circumstances.
Merge pull request #158 from fish-face/patch-1

Correctly find main file when user has non-standard magicness