Simplify cursig() a bit and make sure that signals are always sent to

Simplify cursig() a bit and make sure that signals are always sent to
the parent of ptraced processes. Especially ignore the signal mask set
by sigprocmask(2) in that case. In userret() alter the testcase for
when to call cursig() which is only there to avoid taking the
KERNEL_LOCK when returning from a MP safe syscall. This can be revisited
once cursig() is MP safe.
Problem with debugging signal handlers found by kurt@
Tested and OK kurt@, OK mpi@