On OS X it appears the mach exception thread and libdispatch queue threads occasionally get the SIGALRM from the itimer. Count those as foreign_thread_samples instead of letting the signal kill the process.
Set up the state necessary to collect samples. We still need to add GC support for walking the sample set, and the compiler needs to register GC roots before safepoints as well. We also need primitives to expose the data to Factor for reporting.
Also change the docs around so the useful commands are all listed under "Basic commands", and rename the more arcane "s" and "r" dumps to "ds" and "dr".
We also need to save C ABI volatile registers before calling the signal handler in order to be able to reliably resume. Add signal-handler and leaf-signal-handler subprimitives to preserve volatile registers before invoking the signal handler C function.
This will let us safely unguard the callstack overflow guard page as a grace page so the callstack overflow handler can do its business without discarding frames.
Also rename "q" to "c" because it "c"ontinues, remove the useless "im" command, and rename the less useful "t" to "trim" so we can use "t" to mean "throw"
It's not robust currently to raise an exception because a lot of our code that isn't already written with exceptions in mind breaks. Also, a signal is likely to be received by an FFI callback installed on the IO multiplexer, which will cause Factor to die since the callback cannot handle the exception. We need a more robust solution to dealing with SIGINT.
Also lay some groundwork for counting profile samples and reporting non-interrupting asynchronous signals.
Remove the weird and broken functions fix_callstack_top and scrub_return_address. Instead, simply decrement the SP and store the PC from the front end of the signal handler so that the back end can return back into the original context normally. Currently aborts for leaf procedure frames pending a more robust solution.
Joe Groff