* cygtls.h (san): New structure.

* cygtls.h (san): New structure.
(cygtls::andreas): New element.  Replaces _myfault and _myfault_errno.
(cygtls::fault_guarded): Use andreas.
(cygtls::return_from_fault): Ditto.
(cygtls::setup_fault): Add a parameter denoting where to store old fault
handler, if any and use it to "stack" faults.
(cygtls::reset_fault): Restore fault from parameter.
(myfault::sebastian): New variable.
(myfault::~myfault): Pass sebastian to reset_fault.
(myfault::myfault): Store old fault values in sebastian.