Extend single_thread_set() mode with additional flag attributes.

Extend single_thread_set() mode with additional flag attributes.

The mode can now be or-ed with SINGLE_DEEP or SINGLE_NOWAIT to alter
the behaviour of single_thread_set(). This allows explicit control
of the SINGLE_DEEP behaviour.

If SINGLE_DEEP is set the deep flag is passed to the initial check call
and by that the check will error out instead of suspending (SINGLE_UNWIND)
or exiting (SINGLE_EXIT). The SINGLE_DEEP flag is required in calls to
single_thread_set() outside of userret. E.g. at the start of sys_execve
because the proc is not allowed to call exit1() in that location.

SINGLE_NOWAIT skips the wait at the end of single_thread_set() and therefor
returns BEFORE all threads have been parked. Currently this is only used by
the ptrace code and should not be used anywhere else. Not waiting for all
threads to settle is asking for trouble.

This solves an issue by using SINGLE_UNWIND in the coredump case where
the code should actually exit in case another thread crashed moments earlier.
Also the SINGLE_UNWIND in pledge_fail() is now marked SINGLE_DEEP since
the call to pledge_fail() is for sure not at the kernel boundary.

OK mpi@