fuse/impl_c: reworking of mac/tahoefuse, command line options, test integration

fuse/impl_c: reworking of mac/tahoefuse, command line options, test integration

a handful of changes to the tahoefuse implementation used by the mac build, to
make command line option parsing more flexible and robust, and moreover to
facilitate integration of this implementation with the 'runtests' test harness
used to test the other two implementations.

this patch includes;
- improvements to command line option parsing [ see below ]
- support for 'aliases' akin to other tahoe tools
- tweaks to support linux (ubuntu hardy)

the linux support tweaks are, or at least seem to be, a result of the fact that
hardy ships with fuse 0.2pre3, as opposed to the fuse0.2 that macfuse is based
upon.  at least the versions I was working with have discrepencies in their
interfaces, but on reflection this is probably a 'python-fuse' version issue
rather than fuse per se.  At any rate, the fixes to handling the Stat objects
should be safe against either version, it's just that the bindings on hardy
lacked code that was in the 'fuse' python module on the mac...

command line options:

the need for more flexible invocation in support of the runtests harness led
me to rework the argument parsing from some simple positional hacks with a
pass-through of the remainder to the fuse binding's 'fuse_main' to a system
using twisted.usage to parse arguments, and having just one option '-o' being
explicitly a pass-through for -o options to fuse_main. the options are now:

--node-directory NODEDIR : this is used to look up the node-url to connect
to if that's not specified concretely on the command line, and also used to
determine the location of the cache directory used by the implementation,
specifically '_cache' within the nodedir.  default value: ~/.tahoe

--node-url NODEURL : specify a node-url taking precendence over that found
in the node.url file within the nodedir

--alias ALIAS : specifies the named alias should be mounted. a lookup is
performed in the alias table within 'nodedir' to find the root dir cap
the named alias must exist in the alias table of the specified nodedir

--root-uri ROOTURI : specifies that the given directory uri should be mounted

at least one of --alias and --root-uri must be given (which directory to mount
must be specified somehow)  if both are given --alias takes precedence.

--cache-timeout TIMEOUTSECS : specifies the number of seconds that cached
directory data should be considered valid for.  this tahoefuse implementation
implements directory caching for a limited time; largely because the mac (i.e.
the Finder in particular) tends to make a large number of requests in quick
successsion when browsing the filesystem.  on the flip side, the 'runtests'
unit tests fail in the face of such caching because the changes made to the
underlying tahoe directories are not reflected in the fuse presentation.  by
specifying a cache-timeout of 0 seconds, runtests can force the fuse layer
into refetching directory data upon each request.

any number of -oname=value options may be specified on the command line,
and they will all be passed into the underlying fuse_main call.

a single non-optional argument, the mountpoint, must also be given.