and linux_sendsig functions guarded their debugging output with
ldebug(sigreturn). This has been mistaken for a cut-n-paste bug,
and was replaced by ldebug(rt_sendsig) and ldebug(sendsig) resp.
Since the sendsig functions are not syscalls, this brokei any
build that defines DEBUG.
The fix maps both functions to the unused syscall 0 so that they
can be enabled/disabled independently from sigreturn, but not
independently from each other.