io/sem.t: eliminate warnings

io/sem.t: eliminate warnings

This eliminates some warnings that semctl() (or other *ctl()) calls
might generate, and some warnings specific to io/sem.t:

- for IPC_STAT and GETALL, the current value of ARG is overwritten
  so making an undefined value warning for it nonsensical, so don't
  use SvPV_force().

- for other calls, ARG is either ignored, or in a behaviour
  introduced in perl 3 (along with the ops), treats the supplied
  value as an integer which is then converted to a pointer.  Rather
  than warning on an undef value which is most likely to be ignored
  we treat the undef as zero without the usual warning.

- always pass a number for SEMNUM in the test code

I didn't try to eliminate warning for non-numeric/undefined SEMNUM,
since while we know it isn't used by SETALL, GETALL, IPC_STAT and
IPC_SET, it may or may not be used by system defined *ctl() operators
such as SEM_INFO and SHM_LOCK on Linux.

fixes #17926