Fix --color command line parameter ignorance (#7173)

Fix --color command line parameter ignorance (#7173)

* Fix color command line parameter ignorance

* coloured log: Support detecting the tty on windows

* Print an error message when setting something invalid as color mode instead of silently using mode never

* Revert "coloured log: Support detecting the tty on windows"

This reverts commit 4c9fc6366487ac0e6799e181796ca594797bb6f8.
It didn't work for travis and belongs to a separate PR

* Allow adjusting the log color with an environment variable

If --color is not passed to minetest,  is used to decide on the log colorization.
Minetest settings can not be used instead of an environment variable because logs may appear before loading them.

* fix empty if body