Improve [:alpha:] and [:alnum:] for multibyte characters (Bug#19878)

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:].