add warning about keyfs

Reference: /n/atom/patch/applied/warnkeyfs
Date: Sun Mar 23 15:20:07 CET 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/man/8/auth	Sun Mar 23 15:20:02 2014
+++ /sys/man/8/auth	Sun Mar 23 15:20:03 2014
@@ -290,3 +290,11 @@
 .IR securenet (8)
 .SH BUGS
 Only CPU kernels permit changing userid.
+.PP
+Ensure that
+.I keyfs
+is not running when you run
+.I convkeys
+or
+.IR convkeys2 .
+.PP