Screen/Linux: Fix especially embarrassing compiler warnings.

Found when building against recent gcc’s with all warnings enabled.
These are real bugs, although all of them were dormant so far - not
actually triggered in the real world.
There are tons more spurious warnings, e.g., unused parameter warnings,
which i’ll leave alone for now. Not bored enough.
Update Xcode project file a bit for OS X 10.10.

Just so that there’s less whining from the IDE.