lib/posix-process: Add exit() and exit_group()

lib/posix-process: Add exit() and exit_group()

exit() terminates the calling thread, and the process if it's the last /
only thread in the process. exitgroup() termintates the calling process
leaving it in the zombie state for wait() to reap. for more info see
exit(2) and exit_group(2).

Internally, exit() and exit_group() issue the POSIX_THREAD_EXIT_EVENT
and POSIX_PROCESS_EXIT_EVENT respectively, to notify libraries that have
registered event handlers to these events.

Checkpatch-Ignore: LONG_LINE
Checkpatch-Ignore: MACRO_ARG_REUSE
Signed-off-by: Michalis Pappas <michalis@unikraft.io>
Approved-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Sergiu Moga <sergiu@unikraft.io>
GitHub-Closes: #1630