Make short uuid specification for allow-discards work

Make short uuid specification for allow-discards work

1) strstr " $discarduuids " did not remove the optional
'luks-' prefix from the argument as the documentation says.

2) The lookup seems backwards. $luksdev ($luks in the other code copy)
is the full uuid and thus one should check whether the user-supplied
argument (short form) is contained therein, not the other way around.

Before this commit, the only way to trigger allow-discards was to
specify the full uuid without the 'luks-' prefix.