Provide a MinGW port of 7-Zip

Note: the "--- /dev/null" line, as well as the "+++ ..." line in patch
0006 had to be changed to the path *without* leading a/ and b/ prefixes
because patch.exe would fail to apply the patch otherwise, either
claiming that it has already been applied or patching the file in the
wrong directory...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add mingw32-7z to the master package list

This is necessary if we want to unpack the packages of the MinGW-w64
project (which are all only provided as .7z archives).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>