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 win16 references
Remove NeXTSTEP references
Remove Atari MiNT references
Remove OS/2 references

NOTE: Paul Slootman was removed from the top of os_unix.c but is still
credited somewhere in the docs dir...
Remove Windows 95 references
Remove BeOS/Haiku references