media-sound/pulseaudio: clarify equalizer licensing, handle split speexdsp, remove...

media-sound/pulseaudio: clarify equalizer licensing, handle split speexdsp, remove ancient v6 upgrade handling

PA-6 is well over a year old (with at least a year ago in stable) that needed this upgrade path,
so just remove the REPLACING_VERSIONS handling code instead of fixing it up.
qpaeq is AGPL-3+, so declare it properly. It also needs dbus to get built (the sink module build
requires both HAVE_FFTW and HAVE_DBUS).
Tweak dependencies to pull in split speexdsp when needed.
Also update check dependency minimum version to match configure.ac (albeit we don't have any older
versions in tree).

Gentoo-bug: 589480
Gentoo-bug: 611660