lib/posix-process: Notify process/thread signal files when possible

lib/posix-process: Notify process/thread signal files when possible

Whenever a signal shouldn't be dropped, meaning it is either not ignored by
the process or the process has signal files that are monitoring that
particular signal, notify the existing signal files in question
 of the posix thread/process by setting the input event on said files.

Signed-off-by: Sergiu Moga <sergiu@unikraft.io>
Approved-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Andrei Tatar <andrei@unikraft.io>
GitHub-Closes: #1619