Change handling of default type INSERTED-DEFAULT -- now inserts the

Change handling of default type INSERTED-DEFAULT -- now inserts the
default string only if it is not #f.  Change PROMPT-FOR-STRING/PROMPT
to add ": " to prompt only if it doesn't end in " ".  Change
PROMPT-FOR-NUMBER to display the default without quote marks.  Change
PROMPT-FOR-CONFIRMATION? and PROMPT-FOR-YES-OR-NO? to behave like
Emacs when the user supplies an incorrect answer.