Add support for Git for Windows / MINGW users

If you install Git for Windows (https://git-for-windows.github.io/) you get a console
,either CMD or MINTT, which returns MINGW* for uname -s. So if you run blackbox_addadmin
or the other commands, you'll get: "ERROR: Unknown OS. Exiting."

I've simply added MINGW* to the Linux and CYGWIN checks and it seems to work fine.

NOTE: I didn't touch tools/test_functions.sh allthough it should probably be updated as well.

bin/_blackbox_common.sh

@@ -371,7 +371,7 @@ function md5sum_file() {
     Darwin )
       md5 -r "$1" | awk '{ print $1 }'
       ;;
-    Linux | CYGWIN*)
+    Linux | CYGWIN* | MINGW* )
       md5sum "$1" | awk '{ print $1 }'
       ;;
     * )
@@ -387,7 +387,7 @@ function cp_permissions() {
     Darwin )
       chmod $( stat -f '%p' "$1" ) "${@:2}"
       ;;
-    Linux | CYGWIN* )
+    Linux | CYGWIN* | MINGW* )
       chmod --reference "$1" "${@:2}"
       ;;
     * )