* New script /etc/msystem providing information associated with the
current MSYSTEM. This is sourced by /etc/profile but can be used by
non-login scripts for getting information such as the current
compiler architecture or mount point.
* New script /usr/bin/shell for switching between shells without
restarting MSYS2. This can be used in interactive sessions but also
from shell scripts for switching the MSYSTEM without restarting bash
or spawning a subshell.
* Allow /etc/profile to be sourced multiple times without continuously
appending stuff to PATH or losing track of the original Windows TEMP.
Also implemented for /etc/profile.d/perlbin.sh, so perl path is not
repeatedly appended.
