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.
strings/memory: constify and func_attr.

Fix MB_COPY_cHAR() to accept const pointers.
strings: Enable -Wconvert.