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 crash due to "pure virtual method called" in the "mms" input
plugin.  Closes #253

Without setting the "ready" flag, the caller will wait in WaitReady()
forever, locking up MPD.  Closes #252

Enforcing MADV_DONTFORK is a surprising limitation for this library
which aims to be generic.

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

Fixes deadlock bug in the "mms" plugin.

Replace InputPlugin attributes.

Replaces the method Close().

Prepare for adding virtual methods.