Only a read-only file descriptor is required to implement list_devs()

and list_verbose(), so don't open /dev/pci read-write.  This allows
pciconf -l[v] to work for non-root users, assuming the securelevel is
0 or -1.

Problem experienced by:	William Michael Grim <wgrim@siue.edu>

usr.sbin/pciconf/pciconf.c

@@ -165,7 +165,7 @@ list_devs(int verbose)
 	if (verbose)
 		load_vendors();
 
-	fd = open(_PATH_DEVPCI, O_RDWR, 0);
+	fd = open(_PATH_DEVPCI, O_RDONLY, 0);
 	if (fd < 0)
 		err(1, "%s", _PATH_DEVPCI);