Add Z_SHELL_SET_FLAG_ATOMIC macro to use technique from
shell_process() to atomically set and return the previous
value.
Change all void z_flag_*_set() functions to bool z_flag_*_set()
and use macro to return previous value after setting.
Modify shell_process() to use this modified z_flag_processing_set()
function.
Reorder flags to keep last_nl on byte boundary.
Add public setters for insert_mode, echo, and mode_delete for
completeness.
Signed-off-by: Pete Skeggs <peter.skeggs@nordicsemi.no>
75a74bfad2