macapp: changes to support aliases, updated tahoefuse command line options

macapp: changes to support aliases, updated tahoefuse command line options

the tahoefuse command line options changed to support the runtests harness,
and as part of that gained support for named aliases via --alias

this changes the mac app's invocation of tahoefuse to match that, and also
changes the gui to present the list of defined aliases as valid mounts

this replaces the previous logic which examined the ~/.tahoe/private directory
looking for files ending in '.cap' - an ad-hoc alias mechanism.

if a file is found matching ~/.tahoe/private/ALIASNAME.icns then that will still
be passed to tahoefuse as the icon to display for that filesystem. if no such
file is found, the allmydata icon will be used by default.

the '-olocal' option is passed to tahoefuse.  this is potentially contentious.
specifically this is telling the OS that this is a 'local' filesystem, which is
intended to be used to locally attached devices.  however leopard (OSX 10.5)
will only display non-local filesystems in the Finder's side bar if they are of
fs types specifically known by Finder to be network file systems (nfs, cifs,
webdav, afp)  hence the -olocal flag is the only way on leopard to cause finder
to display the mounted filesystem in the sidebar, but it displays as a 'device'.
there is a potential (i.e. the fuse docs carry warnings) that this may cause
vague and unspecified undesirable behaviour.
(c.f. http://code.google.com/p/macfuse/wiki/FAQ specifically Q4.3 and Q4.1)