This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

Closes #173

Similar to commit 31ab78ae

Fixes deadlock because FileInputStream::Read() unlocks the mutex
(which was not locked) and then locks it, keeping it locked.  This can
result in a deadlock.  This happens because the archive and the file
mutex are different.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

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.