subsequent prompting for occur

[Dan Jacobson <jidanni@jidanni.org>]

When we type M-x occur for a subsequent time, we are prompted with the
regexp we used the last time:
List lines matching regexp (default `^[A-Z]'):

The problem is that we normal people will then try and edit the
expression, which lives in read-only land.

Therefore, kindly prompt like this:
List lines matching regexp: ^[A-Z]
So we can edit it without thinking.

And while you are at it, finally bind "q" to bury the occur buffer.