avoid compiler warnings
fix docs for `image`
unzip: Handle non-seekable input ports.

Before this commit:
  - unzip would attempt to seek the input port via file-position in some cases

After this commit:
  - unzip reads and discards bytes as needed to skip forward
Handle data descriptor signatures in file/unzip

http://www.pkware.com/documents/casestudies/APPNOTE.TXT specifies that a data
descriptor signature may be placed after the compressed data and before the
data descriptor. file/unzip now handles this case.