systemd-cryptsetup: makes “discard” a synonym for “allow-discards”

systemd-cryptsetup recognizes option 'allow-discards' in /etc/crypttab
to enable TRIM passthrough to underlying encrypted device. In Debian
this option was changed to 'discard' to avoid hyphen in option name.
(see: #648868 and `man crypttab`).

[zj: update crypttab(5) too, making "discard" the default.]