Utility for --check-patches

[VRehnberg]

Would be nice with a quick check that patch files are ok. Thought --stop=patch would do what I wanted, but that doesn't touch exts_list or components.

Would probably work similar to however --fetch works but with the extra patch step. Doesn't seem like a small addition and not that important. I'd give it low priority.

Possible alternative usage of such a command when used with --disable-cleanup-builddir would be to prepare the build directory exactly as it would look before the build starts.