exportfs: make -d log to stderr

exportfs: make -d log to stderr

exportfs -d logs 9p traffic to /tmp/exportdb.
-f allows writing to a different file.
exportfs silently continues if it doesn't have
permissions to create or write to /tmp/exportdb.
These are poor behaviors.

A better default is to write to stderr, since it
is 9P debug info that is better immediately printed,
and not user info that is better handled by syslog().
As a result, -f is obsolete and thus removed.
Redirect responsibility is now on rc.
As a side effect, rc will fail if it doesn't
have permissions to write.

exportfs(4) is updated to reflect all changes
and with a better Synopsis.