The hcreate(3) implementation and related functions we inherited
from NetBSD used to free() the key value, something that is not
supported by the standard implementation.
This would cause a segmentation fault when attempting to run
the examples from the opengroup and linux manpages. NetBSD
has added non-standard calls to provide the previous
behaviour but hdestroy is not very commonly used so at this
time it seems excessive to bring those to FreeBSD.
Bump the __FreeBSD_version as this is an ABI change.