[9fans] recover cwfs

[Anthony Sorace <anothy@gmail.com>]

i've got a cwfs based on an old fs(4). it gets used infrequently;
about a month and a haf ago it sufferend a power outage and i just
left it off, since i'd not touched it for a few weeks before that.

today i brought the thing back up, and the active fs is unhappy. on
boot, it reports it can't read /adm/users, which makes mounts fail.
running newuser complains about /adm/users but adds the user to the
table in memor, so i can mount, just to get "phase error -- cannot
happen" on any file access attempt.

accessing the dump file system works just fine. i'm entirely happy to
replace the file system with the last dump. i think "recover" from the
configuration mode is the way to do this, but i can't find a
description of it and it's been years since i had to do it. in
particular, how do i determine the superblock of the last dump?

also, this seems like the right oportunity to start using cwfs's -c.
any notes on the performance differences?