functions: fix title() to not match any $TERM

On my linux virtual terminals, where TERM="linux", I was getting
annoying output that was messing up my prompt.

It turns out the title function was always matching on the elif
statement for xterm/rxvt no matter what and the linux vt doesn't know
what to do with the title special control sequence and thus was printing
out garbage.

Through experimentation I figured out that the || inside of the [[ ]]
did not work:

export TERM=linux
$ if [[ $TERM =~ "^xterm" || $TERM == "rxvt" ]]; then echo $TERM; fi
linux

$ if [[ $TERM =~ "^xterm" ]] || [[ $TERM == "rxvt" ]]; then echo $TERM; fi

Signed-off-by: Brandon Philips <brandon@ifup.org>

openSUSE running zsh 4.3.10