tmux.conf: use tmux-256color

For older systems use tmux-old-ncurses.bash (also added in this commit)
for forcing screen-256color instead.

From what I have understood the difference is that screen-256color
doesn't handle italics or something.

conf/tmux-old-ncurses.bash

@@ -0,0 +1,6 @@
+#!/usr/bin/env bash
+# Intended for systems with ncurses < 6 which is missing TERMINFO
+# for tmux-256color.
+if [[ $TERM == 'tmux-256color' ]]; then
+    export TERM=screen-256color
+fi

conf/tmux.conf

@@ -27,10 +27,10 @@ set -g base-index 1
 bind-key 0 select-window -t :10
 
 # Terminal supports 256colors
-# This should be tmux-256color since ncurses 6.0, consider changing it
-# when ncurses 6.0 hits Debian Stable or is in all systems I have access
-# to.
-set -g default-terminal "screen-256color"
+# In case of older ncurses than 6 / terminfo problems, put the
+# tmux-old-ncurses.bash to ~/.custom or somewhere it gets read on shell
+# startup.
+set -g default-terminal "tmux-256color"
 
 # Terminal emulator window titles
 set -g set-titles on