Replace valid memory checks with new myfault class "exception handling", almost

Replace valid memory checks with new myfault class "exception handling", almost
everywhere.  Leave some thread.cc stuff alone for now.
* cygtls.h: Kludge some definitions to avoid including a problematic windows
header.
(_cygtls::_myfault): New entry.
(_cygtls::_myfault_errno): Ditto.
(_cygtls::fault_guarded): New function.
(_cygtls::setup_fault): Ditto.
(_cygtls::return_from_fault): Ditto.
(_cygtls::clear_fault): Ditto.
(myfault): New class.
* exceptions.cc (handle_exceptions): Handle case of guarded fault in system
routine.
* gendef: Add another entry point for setjmp that the compiler doesn't know
about and won't complain about.
* gentls_offsets: Just include windows.h rather than kludging a HANDLE def.
* miscfuncs.cc (check_null_str): Delete.
(check_null_empty_str): Ditto.
(check_null_empty_str_errno): Ditto.
(check_null_str_errno): Ditto.
(__check_null_invalid_struct): Ditto.
(__check_null_invalid_struct_errno): Ditto.
(__check_invalid_read_ptr): Ditto.
(__check_invalid_read_ptr_errno): Ditto.
(dummytest): New function.
(check_iovec_for_read): Delete.
(chec_iovec): Rename from check_iovec_for_write.  Take a read/write parameter.
* tlsoffsets.h: Regenerate.
* winsup.h: Remove check_* declarations.
(check_iovec_for_read): Delete declaration.  Turn into a define instead.
(check_iovec_for_write): Ditto.
(check_iovec): New declaration.
* thread.h: Use ifdef guard name consistent with other header files.