Add return statements where missing in doio.c IPC functions.

Add return statements where missing in doio.c IPC functions.

These are code paths not seen on platforms that have msg* and
shm* functions, and of course we're croaking before returning
so returning is pointless, but the VMS C compiler insists that
we have return statments for non-void functions anyway, and
that compiler now sees these functions since enabling sem*.