Shortcut cursig when called during sleep setup.

Shortcut cursig when called during sleep setup.

Add deep flag as function argument which is used by the sleep API but
nowhere else. Both calls to sleep_signal_check() should skip the ugly
bits of cursig().

In cursig() if deep once it is clear a signal will be taken keep the
signal on the thread siglist and return. sleep_signal_check() will then
return EINTR or ERESTART based on the signal context.  There is no reason
to do more in this special case. Especially stop/cont and the ptrace trap
must be skipped here. Once the call makes it to userret the signal will be
picked up again and handled in a safe location.

Stopping singals need some additional logic since we don't want to abort
the sleep just to stop a process. Since our SIGSTOP handling requires
a major rewrite this will be posponed until then.

OK mpi@