obsolete Persist config file option and -k flag

This is a leftover relic from early version of burp where a temporary
file was used to store cookies by default. Since we really don't need a
temporary file at all and we can just use an empty string to enable
cookie handling, this functionality can go away.
abstract out cookie domain comparison
fix potential leak on setting aursid
move interactive handling to frontend

This also adds handling to read the username interactively, not just
the password. No reason to get rid of this feature from older burp.