color: Handle NO_COLOR environment variable in default_color_opt()

color: Handle NO_COLOR environment variable in default_color_opt()

The NO_COLOR environment variable is a widely supported way for users
to disable coloured text output.  See <https://no-color.org/>.  In
case iproute2 is configured to use colours by default, allow this to
be overridden by setting NO_COLOR.

This is done in default_color_opt() so that colours can still be
explicitly enabled with a command-line option.

Signed-off-by: Ben Hutchings <benh@debian.org>
Signed-off-by: David Ahern <dsahern@kernel.org>