Changes from Mark Glines in preparation for 0.07

Changes from Mark Glines in preparation for 0.07
- Remove the FUSE_DEBUG constant; we never actually implemented
  it to begin with.
- "make test" now uses the version of Fuse you've just built,
  not the one installed in /usr/lib/perl5.
- getattr test now allows blksize to vary between host and fuse
  fs, as this is not a bug.
- Add experimental support for threading.  The following minor
  API changes accommodate this:
- The nonexistent (yet documented) "unthreaded=>1" attribute
  has been replaced with the "threaded=>1" attribute, and this
  time it actually exists.
- Symbolic refs like "main::e_getattr" are now allowed for
  callbacks, because threaded mode needs to share() the
  callbacks, yet perl 5.8.7 does not allow share()ing code
  refs yet.  Direct code-refs are still supported as much
  as possible (currently, non-threaded mode).
- testsuite uses a multithreaded loopback.pl, when available.
- Update docs accordingly.  Update examples accordingly.

git-svn-id: svn+ssh://llin/home/dpavlin/private/svn/fuse/perl/trunk@19 6e4b0b00-1209-0410-87b2-b275959b5705