lib/uksignal: Retire libuksignal

lib/uksignal: Retire libuksignal

Retire libuksignal in favor of the new implementation introduced
in libposix-process. Stubs of equivalent functionality previously
provided in libuksignal are still available in libposix-process,
when CONFIG_POSIX_PROCESS_SIGNAL is not selected.

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