tor-browser-bundle-bin: optionally disable multiprocess support

Multiprocess tabs always crash, as first reported by the issue mentioned
below.  It is now consistently reproducible both on NixOS and non-NixOS
for me, so I've decided to add a toggle to conveniently disable
multiprocess support as a work-around.

Closes https://github.com/NixOS/nixpkgs/issues/27759 but does
not really fix the underlying problem ...