Omit needless "\ " after multibyte then newline

Omit needless "\ " after multibyte then newline

* src/print.c: Include <c-ctype.h>, for c_isxdigit.
(print_object): When print-escape-multibyte is non-nil and a
multibyte character is followed by a newline or formfeed, followed
by a hex digit, don't output a needless "\ " before the hex digit.
* test/automated/print-tests.el (print-hex-backslash): New test.