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.

manual: remove incorrect paragraph

SELFOUR-279: rename Wait -> Recv, add wrappers for seL4_Poll and seL4_Wait for notification objects.

This commit deprecates seL4_ReplyWait, removes seL4_NBwait completely,
and changes the return type of seL4_Wait to void (seL4_Wait should be
used for notification objects, and seL4_Recv should be used where
seL4_Wait was used previously for endpoints).

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

manual: seL4_Notify -> seL4_Signal; clarifications

- clarified description of `sender` param in Wait/ReplyWait
- updated name of seL4_Notify in API reference
- message word is removed in seL4_Signal
- clarified unbadged/0-badged behaviour in seL4_Signal
- prevent hyphenation inside seL4_Signal

manual: notification/signalling terminology

Terminology clarification:
- synchronous endpoints -> endpoints
- asynchronous endpoints -> notifications (and notification objects)
- notify -> signal

Also minor language edits.

Revert "Terminology fix: s/AEP/Notification/ etc; Draft for review"

Committed to wrong branch

This reverts commit 1c2535c491044ada9bad3c8343d58bae839855cf.

Terminology fix: s/AEP/Notification/ etc; Draft for review

Consistently replaced "asynchronous endpoints" in various guises by notifications,
notify by signal, and removed any refernces to notifications being asynchronous,
as well as removing "synchronous" emphasis of IPC where it existed solely to
contrast with notifications.

This is for review, and thus has comments highlighting major changes and unresolved details.