pbuilder /dev/shm workaround

As pbuilder does not mount a tmpfs onto /dev/shm, I've added an
environment variable, which is capable of overriding default fbr_buffer
file pattern. It is checked only when fbr_context is initialized.
Moved virtual ring buffer implementation into fbr_vrb

Some times one does not need inter-fiber communication pipe, but just a
ring buffer. And using libvrb for this purpose is not really an option.

fbr_buffer internally uses fbr_vrb.