ls: don't output colors with unknown TERM env variable

ls: don't output colors with unknown TERM env variable

--colors controls whether to output colors depending on
whether we're connected to a terminal or not, while this
change gives control over which terminals we output colors to.

* NEWS: Mention the change in behavior.
* src/ls.c (known_term_type): A new function to search the static
list from dircolors.h
(parse_ls_colors): Honor the TERM when both LS_COLORS and COLORTERM
are non empty.
* tests/ls/color-term.sh: A new test.
* tests/local.mk: Reference the new test.
Fixes http://bugs.gnu.org/15992