d/scripts/suspend/suspend.c: Allocate and lock memory for later usage

We have to allocate and lock memory for later usage by LUKS resume
before we LUKS suspend the devices. Otherwise, LUKS resume processes
might cause the system to swap out later, especially in case of LUKS2
devices with argon2i as PBKDF. Swapping out at LUKS resume state would
result in deadlocks if the storage holding swap is suspended.

See the discussion in Gitlab issue #19 for further information:
#19

Besides the mlock(2) and malloc(2) manpages, the following documentation
was helpful for a better understanding of mlock:
* https://eklitzke.org/mlock-and-mlockall
* https://www.informit.com/articles/article.aspx?p=23618&seqNum=9