This is kind of hacky, but it avoids the segfault at runtime. So,
preferrable. If we ever have more options, we need to properly generate
the argv[] array.
Name the flag exactly as it's used by LUKS everywhere: allow_discards
(we had in some places "discard", "allow_discard"). Implement actually
honoring that flag if it's set. Untested code.
Previously, we wrote the passphrase contents to a temporary file on
/dev/shm and then wiped it afterwards. This is odd, why don't we use a
pipe for this purpose, like it's intended to be used? Replace all of
that previous code by piped IPC.
