lib/posix-process: Add wait4() and waitid()

lib/posix-process: Add wait4() and waitid()

wait4() and waitid() block until a state change in the calling process.
In the case of termination, the wait syscall reaps the child. For more
info see wait(2).

This patch implements support for process termination (exit / kill) and
does not implement other state changes like stop and continue.

Checkpatch-Ignore: MACRO_ARG_REUSE
Checkpatch-Ignore: POINTER_LOCATION
Checkpatch-Ignore: TYPECAST_INT_CONSTANT
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