Breakpoint attributes

[Vladimir Prus]

In SVN HEAD, if I create a code breakpoint on function, say main, it's
not saved/loaded for next session -- the 'location' field in breakpoint
table becomes empty. I've tried to fix that, but got somewhat lost.

The Breakpoint class has three fields:

	- m_url
	- m_expression
	- address_

It appears that all of them correspond to the 'location' column in
breakpoint table. Do we need 3 different fields? Especially given
that all of those are to be interpreted by the debugger, and may
have random structure?

- Volodya