replacing all ifndef with pargma once

All the kernel header files now use pargma once rather than the ifndef,
as the pre-processed C files do not change while header files
are protected with pargma once. This will also solve any naming issues
caused by ifndef.

Convert license tags to SPDX identifiers

This commit also converts our own copyright headers to directly use
SPDX, but leaves all other copyright header intact, only adding the
SPDX ident. As far as possible this commit also merges multiple
Data61 copyright statements/headers into one for consistency.

mcs: scheduling context donation over ipc

After this commit, threads blocked on an endpoint can recieve a
scheduling context from the thread that wakes the blocked thread.

mcs: order EP and NTFN queues

Previously IPC signal queues were FIFO, as of this change they are
ordered by priority (FIFO for same prio threads).

SELFOUR-6: Add GrantReply to the rights system.

GrantReply is a new access right added to endpoint capabilities, which
allows seL4_Call to be used on those capabilities (specifically, it
allows reply caps *only* to be granted across endpoints).

Prior to the addition of GrantReply, endpoint capabilities required the
Grant access right, which allowed any arbitrary capabilitiy to be
transferred over an endpoint. Using GrantReply, systems can now be
constructed where threads using seL4_Call over an endpoint do not need to be in the same
security subsystem.

SELFOUR-317: rename async endpoint to notification object, and other
fallout.

Add seL4_NBWait: non blocking wait for notifications and endpoints.

Release snapshot