dircolors enhancement: warn on unknown TERM

[Ed Avis]

The .dir_colors input file for dircolors(1) has a list of terminal 
types that are colorizable.  It would be helpful to also have an 
explicit list in that config file of terminal types that should not be 
colorized.  Then if the value of $TERM is unknown dircolors can print 
a warning.

If you don't want to issue a warning in the general case then at least 
do so when stdout is a tty.  In this day and age all terminals are 
colour-capable and ls(1) assumes this.  Dircolors produces empty 
output on a tty that's not recognized.  But an empty LS_COLORS 
environment variable does not tell ls to avoid colorization, just to 
use the default colour scheme.

--
Ed Avis <address@hidden>