lib/posix-process: Make rt_sitgimedwait stub block

lib/posix-process: Make rt_sitgimedwait stub block

According to rt_sigtimedwait(2) the syscall should return a signal
number on success (> 0) or -1 on failure, so the zero value currently
returned by the stubbed implementation can be misinterpreted by
applications that check against failure. Update the stub to return
implementation to block and never return a signal.

Co-authored-by: Michalis Pappas <michalis@unikraft.io>
Signed-off-by: Marco Schlumpp <marco@unikraft.io>
Signed-off-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Sergiu Moga <sergiu@unikraft.io>
Reviewed-by: Andrei Tatar <andrei@unikraft.io>
Approved-by: Sergiu Moga <sergiu@unikraft.io>
GitHub-Closes: #1639