fuse/impl_b: tweaks from testing on hardy

fuse/impl_b: tweaks from testing on hardy

from testing on linux (specifically ubuntu hardy) the libfuse dll has a
different name, specifically libfuse.so.2. this patch tries libfuse.so
and then falls back to trying .2 if the former fails.

it also changes the unmount behaviour, to simply return from the handler's
loop_forever() loop upon being unmounted, rather than raising an EOFError,
since none of the client code I looked at actually handled that exception,
but did seem to expect to fall off of main() when loop_forever() returned.
Additionally, from my testing unmount typically led to an OSError from the
fuse fd read, rather than an empty read, as the code seemed to expect.

also removed a spurious import pyflakes quibbled about.