cryptsetup now requires dm-crypt v1.18.1 or higher
to use kernel keyring service for passing VKs.
Also, relevant API functions fail if CRYPT_ACTIVATE_KEYRING_KEY
is set, but library is not allowed to use kernel keyring for
VK.
Originally the key description for VK was derived
from segment id. This could lead to ambiguity when
keyslot key is verified and loaded in kernel keyring
using activation functions with CRYPT_ACTIVATE_KEYRING_KEY
flag raised.
On some systems the requested amount of memory causes OOM killer
to kill the process (instead of returning ENOMEM).
For now, we never try to use more than half of available
physical memory.
This patch duplicates part of the code because following
switch to non-recursive automake is not easily fixable without this change.
(Automake cannot use top_srcdir anymore in this context.)
