to 0,0 was unnecessary because wclear() is documented to do that. also,
the immediate wrefresh() done by msg_clear() means that if we can take
advantage of full screen clear sequence, we will.
is _bogus_ in the face of printf-like message formatting! Instead,
calcuate the max size to format when the message window is set. We know
that we'll never want to format more characters than can fit in the window.
which is one too many). Also, msgc manual page says that 'def' and 'val'
can be the same string, but the way input was done (characters typed
went directly into val) meant that the contents of 'def' would be corrupted.
If the user backspaced to the beginning of the line and hit return (to accept
the default), they'd get a combination of the old default string and the
new characters they typed. alloca() a buffer an put new input there to
avoid this problem.
