Fix a crash with prompt strings containing '%'.

Replace a couple of 'printw(str)' with 'printw("%s", str)'.  I otherwise
got a crash on eg prompt_chars("blah%n", "abc"):

  ** %n in writable segment detected ***

  Program received signal SIGABRT, Aborted.
Merge pull request #213 from jugglerchris/master

Fix a crash with prompt strings containing '%'.