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.
netrw: Remove NetReadFixup()

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

(P21) was removed as it doesn't seem to have a point of reference inside
the file, unlike some of the other P# instances.