afsd -dynroot-sparse mode for hushed cells

afsd -dynroot-sparse mode for hushed cells

if an admin specifies -dynroot-sparse (instead of dynroot) come up
with just local cell and cell aliases showing. cell list
is configured as normal. fs newcell works as normal.

document it.

Change-Id: Ie644ed0f9923a85da5451eafaa114ddf36daa671
Reviewed-on: http://gerrit.openafs.org/2217
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>