Add overlay(-O) mount option support

Add overlay(-O) mount option support

Linux supports mounting over non-empty directories by default.
In Solaris this is not the case and -O option is required for
zfs mount to mount a zfs filesystem over a non-empty directory.

For compatibility, I've added support for -O option to mount
zfs filesystems over non-empty directories if the user wants
to, just like in Solaris.

I've defined MS_OVERLAY to record it in the flags variable if
the -O option is supplied.  The flags variable passes through
a few functions and its checked before performing the empty
directory check in zfs_mount function.  If -O is given, the
check is not performed.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #473