ls: --color: honor separate sequences for extension cases

ls: --color: honor separate sequences for extension cases

Following on from commit v8.29-45-g24053fbd8 which unconditionally
used case insensitive extension matching, support selective
case sensitive matching when there are separate extension cases
defined with different display sequences.

* src/dircolors.hin: Document how file name suffixes are matched.
Note this is displayed with `dircolors --print-database` which
the texi info recommends to use for details.
* src/ls.c (parse_ls_color): Postprocess the list to
mark entries for case sensitive matching,
and also adjust so that unmatchable entries are more quickly ignored.
(get_color_indicator): Use exact matching rather than
case insensitive matching if so marked.
* tests/ls/color-ext.sh: Add test cases.
* NEWS: Mention the change in behavior.
Addresses https://bugs.gnu.org/33123