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
REVIEW AND REBASE REVIEW AND REBASE

First run at OS/2

NOTE: Paul Slootman was removed from the top of os_unix.c but is still
credited somewhere in the docs dir...
runtime/autoload/netrw.vim: rm 'has("win95")'
netrw: Remove NetReadFixup()

If what NetReadFixup() fixes actually still happens then it should
arguably be fixed in the protocol, not in vimscript. Besides, it's
apparently only applicable to win95 (which is unsupported).
netrw docs: Remove NetReadFixup() references.