Fixes assertion failure.

mixramp_delay==nan() causes severe problems with cross-fading.

2 MB was too small for cross-fading a 24 bit file.  Increasing to 4 MB
is still not too large for weak machines, but is enough for
cross-fading.

Without -Werror, clang will not fail on -ftree-vectorize.

Allows advanced compiler optimization.

Most importantly: always disable C++ exceptions.  It was surprising to
see MPD terminate due to an unexpected exception.

The last piece to allow -ffast-math.

Avoid NaN to allow -ffast-math.

Use a boolean flag instead.

Avoid NaN to allow -ffast-math.

Avoid NaN to allow -ffast-math.

Fixes crash.

Restores the features from the previous draft commands "findin" /
"searchin".

This reverts commit a577944a.

Will be replaced by new options for the old commands "search" and
"find".

According to the protocol documentation, matching the file name was
wrong.  This removes some awkward special-case code.