i#2907: delay itimers until handlers are in place

With the new gap between init and attach where we have no signal handlers, we have to delay setting up itimers (such as for -prof_pcs) until we have our handlers ready.

Tested via a custom change to api.static_signal documented in the issue. That change is included here but with a smaller delay to reduce test suite impact.

Fixes #2907 (closed)