Improve [:alpha:] and [:alnum:] for multibyte characters (Bug#19878)
src/character.c (alphabeticp, decimalnump): New functions.
src/character.h (alphabeticp, decimalnump): Add prototypes.
src/regex.c (ISALNUM, ISALPHA): Check Unicode character properties
for multibyte characters by calling alphabeticp and decimalnump.
(BIT_ALPHA, BIT_ALNUM): New bit masks.
(re_wctype_to_bit): Return them when the class is RECC_ALPHA or
RECC_ALNUM.
(re_match_2_internal): Call ISALPHA and ISALNUM when appropriate.
doc/lispref/searching.texi (Char Classes): Update the documentation of
[:alpha:] and [:alnum:].
etc/NEWS: Mention the changes in [:alpha:] and [:alnum:].