ls: avoid reading beyond end of color indicator

ls: avoid reading beyond end of color indicator

At first this looked like a buffer overrun, since there was no test
to ensure that the buffer length was 6.  However, since the LS_COLORS
string is NUL-terminated and since settings within it are separated by
":" there was neither the risk of reading beyond end of buffer nor risk
of a false-positive match.
* src/ls.c (print_color_indicator): Use color_symlink_as_referent
rather than manually comparing against "target" again.
* src/system.h (STRNCMP_LIT): Correct description in comment.