exportfs, oexportfs, iostats: make -d log to stderr

exportfs, oexportfs, iostats: 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As a result, -f is obsolete and thus removed.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.

oexportfs is changed to match exportfs.
oexportfs(4) is updated to reflect all changes.
The Synopsis is not changed due to the number of flags.

Removed -f from iostats.
iostats(4) is updated to reflect all changes.
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