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.

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.

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

Replace InputPlugin attributes.

Replaces the method Close().

Prepare for adding virtual methods.

Merge some duplicate code.