This patch makes `unzip' mmap the zip file instead of seeking the file
repeatedly.
This makes unzipping big files bearable, and unzipping huge files
impossible.
The later could be fixed by mapping/unmapping chunks on the fly once we
have support for mmap() offsets.
The unzip command will unzip a zip file passed as an argument into the
current pwd, with the syntax:
unzip file.zip
This implementation is pretty barebones as it does not support things
like file access times, compression or even compression detection, so
if the user tries to unzip a compressed zip most probably he would find
wrong data inside the files.
However it's an starting point :^)
