Fixes unmount when imported readonly=on.

Merges some of 0fc19e7637f2d04492f1e5e0cea5649cbbf0dcc9
in the ctldir branch, so that we can export pools that
were imported zpool import -o readonly=on.

Unfortunately the fix is stripping out the OS X specific
"modified time" (distinct from mtime on OS X).

Support for modified time was originally added to the
legacy MacZFS in commit:

https://code.google.com/p/maczfs/source/diff?spec=svnf25aceff09b1024a6fc9cada437fbfe4254e58de&r=f25aceff09b1024a6fc9cada437fbfe4254e58de&format=side&path=/zfs-119/zfs_kext/zfs/zfs_vfsops.c