unbind_to performance tuning

unbind_to performance tuning

* src/alloc.c (which_symbols):
* src/dispnew.c (Fredisplay):
* src/editfns.c (Fsubst_char_in_region):
* src/fileio.c (Fdo_auto_save):
* src/indent.c (Fvertical_motion):
* src/keymap.c (Fcurrent_active_maps):
* src/lread.c (Feval_buffer):
* src/minibuf.c (get_minibuffer):
* src/sysdep.c (system_process_attributes):
* src/textprop.c (Fnext_single_char_property_change)
(Fprevious_single_char_property_change):
* src/window.c (Fscroll_other_window, Fscroll_other_window_down):
* src/xdisp.c (Fformat_mode_line):
Help the compiler eliminate tail recursion in call to unbind_to.
* src/coding.c (decode_coding_gap):
Omit unnecessary unbind_to, as we’re about to call unbind_to anyway.
* src/coding.c (Fread_coding_system):
* src/eval.c (eval_sub):
* src/xdisp.c (handle_single_display_spec, decode_mode_spec):
* src/xselect.c (x_get_local_selection):
Avoid need to save a machine register when calling unbind_to.
* src/minibuf.c (Ftry_completion, Fall_completions):
Omit unnecessary assignment.