destroyed NFS exported filesystems not removed from /etc/zfs/exports

[Kris Kennaway]

mountd was failing to start after an upgrade, with lots of log entries like:

Jun 17 10:19:22 <daemon.err> pointyhat mountd[855]: bad exports list 
line /a/portbuild/i386/20090421213917
Jun 17 10:19:22 <daemon.err> pointyhat mountd[855]: bad exports list 
line /a/portbuild/i386/7-exp/builds/20090422073914/src
Jun 17 10:19:22 <daemon.err> pointyhat mountd[855]: bad exports list 
line /a/portbuild/i386/8-exp/builds/20080814181849/ports

These came from /etc/zfs/exports, and refer to filesystems that used to 
exist & were exported, but which have been destroyed.  These were not 
cleaned up at destroy time.

zfs unshare -a didn't clean this file up either, I had to remove it and 
recreate.

Also, mountd was treating these as fatal errors at runtime and failing 
to start.  I think this is a recent change in mountd, since this used to 
work even with the stale entries (the 200808 filesystem was destroyed in 
2008, and mountd has started correctly after numerous reboots until the 
upgrade I just did from a May 9 world+kernel).

Kris