Add a mount command

Remove the fork from cs, it's breaking akaros threading somehow.

So what should work:

ash ifconfig
cs&
mount -n -b '#s/cs' /net
csquery
(type commands)
but, ah, well, no. i don't see /9/net/cs there.

Signed-off-by: Ronald G. Minnich <rminnich@google.com>