Simplify control input definitions.

Simplify the control input definitions in rmfbd_* and mbstereophony by avoiding
"selector".  This drastically reduces compilation time: rmfbd_syn takes about 13
minutes vs. about 60 minutes before.
Split mbstereophony into four versions a la rmfb*.

Since Yann Orlarey pointed me to a way to simplify the frequency slider
definitions, it is now feasible to create versions of mbstereophony with
variable edge frequencies.  Therefor, split it into four versions like the rmfb*
programs.
Update the README.
SCons: support FAUST's work stealing scheduler.
Do not define "mydsp".
README: cite the paper for the TF invariance claim.