Fix: common: improve color support handling

With this patch, terminal color codes are emitted by Babeltrace modules
if the `BABELTRACE_TERM_COLOR` environment variable is set to `ALWAYS`
or if all the following conditions are satisfied:

1. The `BABELTRACE_TERM_COLOR` environement variable is not set to
   `NEVER`.
2. The `TERM` environment variable starts with `xterm`, `rxvt`,
   `konsole`, `gnome`, `screen`, `tmux`, or `putty`.
3. For both the standard output and error streams: its file descriptor
   is a TTY.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>