* syscall:
	add syscall.RawSyscall, which doesn't use sys.entersyscall/sys.exitsyscall
	add syscall.dup2
	add syscall.BytePtrPtr
	add syscall.Rusage, RusagePtr
	add syscall.F_GETFD, F_SETFD, FD_CLOEXEC

* runtime:
	clean up, correct signal handling.
	can now survive (continue running after) a signal.

R=r
DELTA=394  (286 added, 51 deleted, 57 changed)
OCL=20351
CL=20369