CMake: Set -Wconversion by default.
Wconversion: Fix warnings in cursor_shape.c.
Wconversion: Fix warnings in digraph.c.
Merge pull request #1488 from fwalch/invert-wconversion

Invert -Wconversion handling & fix some warnings.
Remove NeXTSTEP references

Remove trailing whitespace as well.
Remove Atari MiNT references
Remove OS/2 references

Paul Slootman was removed from the top of os_unix.c as OS/2 is no longer
supported, but is still credited in runtime/doc/intro.txt.