mount: implement -o nosymfollow, remove bogus -o union

The (1 << 8) MS_ flag is MS_NOSYMFOLLOW, not MS_UNION.
As far as I see in git history of kernel and util-linux,
MS_UNION did not ever exist.
Why did it appear in our tree in 2009?...

function                                             old     new   delta
mount_option_str                                     379     385      +6

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
1 file changed