crosh: update default TERM to 256color

The default xterm doesn't provide us with extra fun colors, so update
the default to allow that.  This keeps us in sync w/hterm too.

Bug: 800049
Test: compiled & ran it
Change-Id: I648ec77ab5adc8542d2673848f666e5e8568db8b
Reviewed-on: https://chromium-review.googlesource.com/905264Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556736}

chromeos/process_proxy/process_proxy.cc

@@ -238,7 +238,10 @@ int ProcessProxy::LaunchProcess(const base::CommandLine& cmdline,
   options.allow_new_privs = base::CommandLine::ForCurrentProcess()->
       HasSwitch(chromeos::switches::kSystemInDevMode);
   options.ctrl_terminal_fd = slave_fd;
-  options.environ["TERM"] = "xterm";
+  // TODO(vapier): Ideally we'd just use the env settings from hterm itself.
+  // We can't let the user inject any env var they want, but we should be able
+  // to filter the $TERM value dynamically.
+  options.environ["TERM"] = "xterm-256color";
   options.environ["CROS_USER_ID_HASH"] = user_id_hash;
 
   // Launch the process.