ls: handle disabling of colors consistently for all tile types

ls: handle disabling of colors consistently for all tile types

* src/ls.c (print_color_indicator): Use consistent syntax for
all file and directory subtypes, and fall back to the color
of the base type if there is no enabled color for the subtype.
This allows turning off specific colors for o+w dirs for example.
* tests/ls/color-dtype-dir: Add a case to test that turning off
coloring for o+w directories, falls back to standard dir color.
* NEWS: Mention the fix
Introduced by commit ac467814, 2005-09-05,
"Colorize set-user-ID ... files and sticky ... directories."