Initial support for handling signals delivered to the syscallbuf lib.

This is *very* basic support code-wise, but I think should document the edge cases reasonably well (and abort on the unhandled ones).  The code was pretty straightforward except that it turns out ptrace doesn't honor `PTRACE_O_TRACESYSGOOD` for `PTRACE_SINGLESTEP`.  I lost about an hour rewriting the syscallbuf code to make trace-traps through a new traced helper, when I realized that linux reports `int 0x80` as a breakpoint-trap signal instead of a trace-trap signal.  Oh well, I saved the WIP patch.