This fixes an old bug which caused the "unused" warnings to be
unreliable; only the first block in the list was marked as being
"used", no matter if it was really used, and the rest was never marked
as "used", suppressing all warnings for them.

And specify whether generic tags are being scanned.

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.

Instead of setting state=IGNORE_TRACK, ignore only the following
"INDEX" lines.

Correction for commit 8461d71b.  Closes #227

Fixes a deadlock caused by commit
31ab78ae.  That commit was not
actually bad - just these two calls have always been bad, which went
unnoticed for a long time.

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

This code (added 7 years ago with commit b233c145) has been obsoleted
by the SongLoader class (added 3 years ago).