ls: don't output any colors with TERM=dumb

The TERM variable is usually set to "dumb" to indicate that the terminal
does not support any ANSI escape sequences. Presently, ls does not honor
this variable and outputs colors anyhow which results in unreadable
output, unless the user explicitly disables colors using `ls
--color=never`. The rational behind this change is that ls should "just
work" by default, even on dumb terminals.

For this reason, this patch adds a check which additionally consults the
TERM variable before printing any colors. This is analogous to the
existing check for ensuring that standard output is a tty. As such,
colors can still be forced with `--color=force`, even if TERM is set to
dumb.

function                                             old     new   delta
is_TERM_dumb                                           -      40     +40
ls_main                                              579     598     +19
.rodata                                           103246  103251      +5
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 2/0 up/down: 64/0)               Total: 64 bytes

Signed-off-by: Sören Tempel <soeren+git@soeren-tempel.net>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

include/libbb.h

@@ -1773,6 +1773,7 @@ extern void print_login_issue(const char *issue_file, const char *tty) FAST_FUNC
 extern void print_login_prompt(void) FAST_FUNC;
 
 char *xmalloc_ttyname(int fd) FAST_FUNC RETURNS_MALLOC;
+int is_TERM_dumb(void) FAST_FUNC;
 /* NB: typically you want to pass fd 0, not 1. Think 'applet | grep something' */
 int get_terminal_width_height(int fd, unsigned *width, unsigned *height) FAST_FUNC;
 int get_terminal_width(int fd) FAST_FUNC;