This also fixes the mishandling of "\N{CJK COMPATIBILITY
IDEOGRAPH-F900}", "\N{VARIATION SELECTOR-1}", etc.
Problem reported by Eli Zaretskii in:
http://lists.gnu.org/archive/html/emacs-devel/2016-04/msg00614.html
* doc/lispref/nonascii.texi (Character Codes), etc/NEWS: Document this.
* lisp/international/mule-cmds.el (char-from-name): New function.
(read-char-by-name): Use it. Document that "BED" is treated as
a name, not as a hexadecimal number. Reject out-of-range integers,
floating-point numbers, and strings with trailing junk.
* src/lread.c (character_name_to_code): Call char-from-name
instead of inspecting ucs-names directly, so that we handle
computed names like "VARIATION SELECTOR-1". Do not use an auto
string, since char-from-name might GC.
* test/src/lread-tests.el: Add tests for new behavior, and
fix some old tests that were wrong.