Add customizable to display raw bytes as hex

Add customizable to display raw bytes as hex

* src/xdisp.c (get_next_display_element): Dispatch used format string
for unprintables based on new display-raw-bytes-as-hex variable.
(display-raw-bytes-as-hex): New variable.  (Bug#27122)

* lisp/cus-start.el: Add defcustom form for display-raw-bytes-as-hex.

* doc/emacs/display.texi: Document the new variable.
* etc/NEWS: Mention display-raw-bytes-as-hex.

* test/manual/redisplay-testsuite.el (test-redisplay-5-toggle)
(test-redisplay-5): New tests.
(test-redisplay): Call test-redisplay-5.