Summary: ftpfs caching behavior seems mostly undesirable.  This patch
eliminates caching by default and allows the previous behavior to be
restored with a -h flag.

Details: the ftpfs manpage currently states "Ftpfs caches files and
directories.  A directory will fall from the cache after 5 quiescent
minutes or if the local user changes the directory by writing or
removing a file." - but I believe this is not accurate.  According to
my testing, there is no time-based directory cache expiry.  The
ftpfs.h file does define an ISOLD macro that would fit the manpage,
but a grep reveals that the ISOLD macro is never used anywhere in the
current code.  The cache can be cleared by using the special
.flush.ftpfs directory entry.

Right now, the 9gridchan public grid is using ftpd/ftpfs to store
files with more rapid upload/download than are available from the 9p
ramfs service.  Clients also transmit messages via a shared plumber to
cause other connected users to view images and documents.  The current
ftpfs behavior clashes badly with this mode of usage - someone uploads
a file, and no one else can see it and it cannot be plumbed
until/unless clients and the plumber hit the .flush.ftpfs.

Given modern networks and machines, the caching seems unnecessary even
in usage situations other than the public grid.  From looking at the
code, the only complication I see might be the the way the caching is
used to create synthetic top directories for the VM and VMS operating
systems in proto.c.

The patch is implemented crudely by finding all the uses of the
ISCACHED macro in the code and making them conditional on the -h
docache flag.  This leaves the calls to uncache() and uncachedir()
unchanged but I don't believe this has any negative consequences.  The
patch has been only lightly tested and seems to do the trick as
desired, but I haven't studied the code or tested extensively enough
to guarantee that there aren't any other side effects.

If people think the current behavior should remain the default, with
the flag being used to turn off caching, that seems just as good to me
and the patch could remain the same except flipping the polarity of
the tests and changing the var name from 'docache' to 'nocache' and
changing my edit to the manpage to reflect this.

In general ftpfs is a crufty relic of an earlier era, but we are
getting some use from it on the public grid, so I thought I'd put this
forward.
-mycroftiv