Clean up var watcher disabling on thread switching

Clean up var watcher disabling on thread switching

* src/data.c (Fset_default): Move code into new C level function,
`set_default_internal'.
(set_default_internal): New function, like `Fset_default' but also takes
additional bindflag parameter.
(set_internal): Only call `notify_variable_watchers' if bindflag is not
SET_INTERNAL_THREAD_SWITCH.
* src/eval.c (do_specbind, do_one_unbind): Add bindflag parameter,
passed on to set_internal and set_default_internal.  Adjust callers.
(rebind_for_thread_switch, unbind_for_thread_switch): Pass
SET_INTERNAL_THREAD_SWITCH to do_specbind, do_one_unbind instead of
temporarily adjusting symbol's trapped_write field.