Calculate memory usage for resume process

The OOM killer issue that's mentioned in #19 (closed) most likely is due to the fact that unlocking LUKS2 devices with argon2 as KDF eats a lot of memory. According to @guilhem, argon2 memory usage is capped at 1G. @guilhem, do you have a reference?

In any case, we should check whether one of the to-be-processed devices is a LUKS2 device with argon2 as KDF and if there's enough memory available.