macfuse: rework fuse initialisation, integrate with 'tahoe'

macfuse: rework fuse initialisation, integrate with 'tahoe'

this provides a variety of changes to the macfuse 'tahoefuse' implementation.
most notably it extends the 'tahoe' command available through the mac build
to provide a 'fuse' subcommand, which invokes tahoefuse.  this addresses
various aspects of main(argv) handling, sys.argv manipulation to provide an
appropriate command line syntax that meshes with the fuse library's built-
in command line parsing.

this provides a "tahoe fuse [dir_cap_name] [fuse_options] mountpoint"
command, where dir_cap_name is an optional name of a .cap file to be found
in ~/.tahoe/private defaulting to the standard root_dir.cap. fuse_options
if given are passed into the fuse system as its normal command line options
and the mountpoint is checked for existence before launching fuse.

the tahoe 'fuse' command is provided as an additional_command to the tahoe
runner in the case that it's launched from the mac .app binary.

this also includes a tweak to the TFS class which incorporates the ctime
and mtime of files into the tahoe fs model, if available.