Avoid wasting 4 kB stack per directory level.

The check was added in commit 98fd9b7d, but there was never a
definition.

Set the Error when zzip_seek()==-1 and not on success.  Fixes a crash
after seeking.

Simplify the definition of many build options.

Has been moved to class InputStream long ago.

Remove the "whence" parameter that is not actually necessary, and only
complicates the InputStream implementations.

Replace InputPlugin attributes.

Replaces the method Close().

Prepare for adding virtual methods.

Merge some duplicate code.