lib/posix-process/signal: Add clone handler for signals

lib/posix-process/signal: Add clone handler for signals

Migrate the clone handler into posix-process/signals.
Implement inheritance of signal handlers.

For more info see CLONE(2)

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