signal handling

[Ryan Hileman]

signal(), sigaction() on posix. when a signal is issued, u.Trampoline() to the dest

what if a signal never returns, then another signal is issued? trampolines can trampoline, but gdb will break... I think the main loop needs to be a trampoline too

trampolines should be "run contexts" and keep track of whether they need to keep going as well as setup/cleanup routines
who gets to clean up on exit? maybe have separate OnExit