Make simta_waitpid() semantics match what callers expect
Make simta_waitpid() semantics match what callers expect

The loop in the function is fine when called with WNOHANG, but callers
don't expect a call to wait for every single child to finish, which
is what happened without WNOHANG. Receive children especially want a
chance to launch a new Deliver child once one has finished.

simta/simta,simta/simta,simta/simta