Make the LinuxKPI task struct persistent accross system calls.

A set of helper functions have been added to manage the life of the
LinuxKPI task struct. When an external system call or task is invoked,
a check is made to create the task struct by demand. A thread
destructor callback is registered to free the task struct when a
thread exits to avoid memory leaks.

This change lays the ground for emulating the Linux kernel more
closely which is a dependency by the code using the LinuxKPI APIs.

Add new dedicated td_lkpi_task field has been added to struct thread
instead of abusing td_retval[1].

Fix some header file inclusions to make LINT kernel build properly
after this change.

Bump the __FreeBSD_version to force a rebuild of all kernel modules.

MFC after: 1 week
Sponsored by: Mellanox Technologies