fix title setting bug in xterm and urxvt

Before this patch, commands containing %-signs set the title wrong
(urxvt and xterm) [1] and produce strange output in urxvt [2].

test for bug 1:
> sleep 10 && echo %
sets title to "sleep 10 && echo %<<"
> sleep 10 && echo %f
doesn't change the title at all

test for bug 2 (only urxvt):
> echo %f
39m%f
^^^^^----- output