FastJar

This patch tries to address bug 200324. FastJar gives an error when listing
the contents of some jar files, even though it extracts them correctly. The
problem appears to occur in archives with a zipfile comment at the end. In
this case the central-header-end section is not 22 bytes from the end of
the file as the list_jar() function in jartool.c expects.

An alternative method of finding the central header is already used for
non-seekable files (e.g. when reading a jar file from stdin). So this patch
simply falls back on the method used for non-seekable files when reading
the central-header-end section does not work. It fixes the problem for me.

The non-seekable files case is also changed to exit with an error if the
file being read appears to have an invalid format.

The patch looks big only because I needed to re-indent a block of code.