This filesystem checks signatures on the files in it.  If the
signatures do not come from a trusted source, then an EACCES is
returned on any read or open attempts.

To make this function, a file system hierarchy with detached
signatures is needed:

	[23:52:54] agc@inspiron1300 ~ 79 > ls -al ~/signed/bin
	total 266
	drwxr-xr-x  2 agc  agc     512 Mar 25 23:52 .
	drwxr-xr-x  3 agc  agc     512 Mar 25 10:35 ..
	-rw-r--r--  1 agc  agc  125659 Mar 25 22:50 pax
	-rw-r--r--  1 agc  agc     280 Mar 25 23:45 pax.sig
	-rw-r--r--  1 agc  agc  135493 Mar 25 22:51 sh
	-rw-r--r--  1 agc  agc     280 Mar 25 22:54 sh.sig
	[23:53:03] agc@inspiron1300 ~ 80 >

The signedfs file system should then be mounted:

	[23:53:59] agc@inspiron1300 ...examples/refuse/signedfs 134 > priv ./signedfs ~/signed/bin /mnt

and then, in a different window, we can access it:

	[23:55:20] agc@inspiron1300 ~ 81 > /mnt/sh
	$ w
	11:55PM  up 10:38, 0 users, load averages: 0.04, 0.05, 0.00
	USER     TTY     FROM              LOGIN@  IDLE WHAT
	$ uptime
	11:55PM  up 10:38, 0 users, load averages: 0.04, 0.05, 0.00
	$ exit
	[23:55:42] agc@inspiron1300 ~ 82 >

If we now change the signature file so that it cannot match the binary:

	[23:56:49] agc@inspiron1300 ~ 83 > /mnt/pax -v -z -f /usr/sets/20070311/sets/xetc.tgz
	/mnt/pax: Permission denied.
	[23:57:02] agc@inspiron1300 ~ 84 >

Verification messages are printed on the window where signedfs was started.
