gdb: add breakpoint "has locations" methods

Add three convenience methods to struct breakpoint:

 - has_locations: returns true if the breakpoint has at least one
   location
 - has_single_location: returns true if the breakpoint has exactly one
   location
 - has_multiple_locations: returns true if the breakpoint has more than
   one location

A subsequent patch changes the list of breakpoints to be an
intrusive_list, so all these spots would need to change.  But in any
case, I think that this:

  if (b->has_multiple_locations ())

conveys the intention better than:

  if (b->loc != nullptr && b->loc->next != nullptr)

Change-Id: Ib18c3605fd35d425ef9df82cb7aacff1606c6747
Reviewed-By: Andrew Burgess <aburgess@redhat.com>