cryptsetup support a "-" (dash) value to the --key-file option.
mount.crypt can already do the same passing keyfile=/dev/stdin
Although, while keyfile vs key-file is a first inconsistency between both binaries, it would nice anyway if mount.crypt could support the "-" option as well.
This is what this patch does. Please note that the documentation does not say anything about passphrase prompting and isatty(). For that reason, we just currently don't support interactive passphrase input.