api: Cygwin compatibility

[Artoria2e5]

This change adds Cygwin compatibility by generalizing some Windows and Linux functionalities. In particular:

• Some tests for Windows are changed into one that consider and accept Cygwin/MSYS2 runtime layers.
• Registry values no longer change magick_home; they now influence PATH instead. ctypes on (plain) Windows search based on PATH; on Unixized Windows they use the normal *nix stuff.
• msvcrt-specific parts are not changed, so they still test for plain Windows.
• *nix libc loading now default to the BSD "let ctypes find libc" loader, technically correct for every *nix system including Darwin. The hardcoded libc6 value is preserved as a fallback.
• *nix libmagick handling now accepts a separate library as found on Cygwin and MinGW.

[Artoria2e5]

ping… would anyone take a look at this? It works for me last time I checked.