BLD: Change `cyg2win32` to use the `cygpath` utility.
BLD: Change `cyg2win32` to use the `cygpath` utility.

The old function only handled the default `/cygdrive` prefix.  This
can be customized to different values: `/` and `/mnt` are both common.
`/proc/cygdrive` does the same thing, regardless of user customization.

`cygpath` handles all of these.  There's also a C function if you'd
prefer to avoid the `fork()` call, which tends to be slow on Cygwin.

Edit: Fix the cyg2win32 function to have correct types.

It returned bytes earlier; this should return a string.

Edit: Fix docsrting to follow numpydoc.

BLD,DOC: Expand documentation for cyg2win32

Fix a misleading statement, and expand on the things `cygpath`
handles that can otherwise be tricky to get right.

simongibbons/numpy,numpy/numpy,seberg/numpy,pdebuyl/numpy,seberg/numpy,endolith/numpy,charris/numpy,mattip/numpy,mhvk/numpy,endolith/numpy,mhvk/numpy,charris/numpy,pdebuyl/numpy,endolith/numpy,pdebuyl/numpy,rgommers/numpy,rgommers/numpy,anntzer/numpy,charris/numpy,mhvk/numpy,mattip/numpy,pdebuyl/numpy,mhvk/numpy,jakirkham/numpy,jakirkham/numpy,simongibbons/numpy,simongibbons/numpy,jakirkham/numpy,anntzer/numpy,endolith/numpy,simongibbons/numpy,jakirkham/numpy,simongibbons/numpy,anntzer/numpy,numpy/numpy,jakirkham/numpy,seberg/numpy,charris/numpy,mattip/numpy,mhvk/numpy,mattip/numpy,seberg/numpy,rgommers/numpy,numpy/numpy,numpy/numpy,anntzer/numpy,rgommers/numpy