Fix OPENAL knob in multimedia/qt5-multimedia

The knob was already there, but didn't have any machinery attached.

• Move USES=openal to the knob
• Introduce a patch to actually make openal configurable (instead of only following autodetect, so you can still turn the knob off with OpenAL installed)
• Tidy the port a little
• Add OPENAL to default options.

Previously, setting or unsetting OPENAL had no effect, and OpenAL
was always a dependency and used. Now, it does have an effect.
I've made it default so that the default options still match
what the port previously did (that is, depend on OpenAL).
Bump PORTREVISION because of that.

(Based very loosely on a patch submitted by Piotr Smyrak)

PR: 242595
Reported by: Piotr Smyrak