Support for MSYS2.
Fix temporary paths in both MSYS and MSYS2.

  * $ORIGINAL_TEMP isn't actually exported from /etc/profile in MSYS2,
    switching to $temp.

  * MSYS actually does define $MSYSTEM, so we now rely on the absence of $temp
    for identifying it (hopefully this is cheaper than checking the version with
    `uname -r`).
Merge pull request #83 from renatosilva/vimcat-msys2

Support for MSYS2.