ogle: add BreakpointAtFunction and BreakpointAtLine, for setting a breakpoint at the start of a function or at a particular source line.

Breakpoint now only takes an address, instead of a formatted string to be evaluated.

Change-Id: I148960dcc149a7467478590eb7c6696597168a03
Reviewed-on: https://go-review.googlesource.com/10624
Reviewed-by: Rob Pike <r@golang.org>