Sync .tmux.conf, resetting default term to xterm.
Revert "Sync .tmux.conf, resetting default term to xterm."

This reverts commit 1263c5a00ac68dbde0ab7416ccffe7e33a3e24d2.
Sync xmonad.hs, setting a command to require tmux, passing -2u to force 256 colors and utf-8 mode.

Part of configuring automatic requiring of tmux sessions on new terminals.
Sync xmonad.hs, where now ctrl+enter functions as a backup terminal starter; also adds second terminal fallback.

These fallbacks can be useful when automatically starting a tmux session breaks
and normal terminals fail to start, whether for a protocol mismatch or other
reason.