lib/posix-process/signal: Add signal delivery

lib/posix-process/signal: Add signal delivery

Implement signal delivery. Signals are delivered at syscall exit
by registering a handler to the syscall shim. The same handler
could be potentially register a handler to a preemptive scheduler's
context switch.

Notable differences from Linux:
- The Core disposition falls back to Term, as we don't support
  application core dumps.
- The Stop and Cont dispositions are ignored, as these are normally
  relevant to shells.

Checkpatch-Ignore: LONG_LINE
Signed-off-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Ioan-Teodor Teugea <ioan_teodor.teugea@stud.acs.upb.ro>
Reviewed-by: Sergiu Moga <sergiu@unikraft.io>
Reviewed-by: Andrei Tatar <andrei@unikraft.io>
Approved-by: Andrei Tatar <andrei@unikraft.io>
GitHub-Closes: #1248