Wrapper for running uRXVT-X in specific directory

This allows us to run uRXVT-X from Windows File Explorer context menu.
Registry entries are provided.
Correcting systemName() for MinGW introducing mingwRootDir()

Calling "uname -o" on MinGW resulted in unrecognized option, so falling
back to printing 'MinGW' and 'Cygwin' for their respective environments.

Function mingwRootDir() returns MinGW root directory in Windows
(alternative, i.e. separated by '/') format. This can be used when
determining if it's Msys Git environment.
PS1 modification for Msys and Msys Git environment