expr: add multibyte support

expr: add multibyte support

Discussed in https://bugs.gnu.org/26779 .

* NEWS: Mention the improvement.
* bootstrap.conf: Add gnulib modules mbslen,mbschr.
* src/expr.c (mbs_logical_substr): New function to return a substring
based on logical character positions (instead of bytes).
(mbs_logical_cspn): Similar to strcspn/mbscspn, but returns number of
logical characters instead of byte offset.
(mbs_offset_to_chars): New function to return number of logical
characters fitting in a given byte offset.
(docolon): Report matched logical characters instead of bytes.
(eval6): For length/substr/index operations, use logical characters
instead of bytes by calling the above new functions.
* tests/misc/expr.pl: Repeat all tests with non-C locale to detect any
regressions.
* tests/misc/expr-multibyte.pl: New tests with multibyte input.
* tests/local.mk: Add new test file.