gdb: use -1 for breakpoint::task default value

gdb: use -1 for breakpoint::task default value

Within the breakpoint struct we have two fields ::thread and ::task
which are used for thread or task specific breakpoints.  When a
breakpoint doesn't have a specific thread or task then these fields
have the values -1 and 0 respectively.

There's no particular reason (as far as I can tell) why these two
"default" values are different, and I find the difference a little
confusing.  Long term I'd like to potentially fold these two fields
into a single field, but that isn't what this commit does.

What this commit does is switch to using -1 as the "default" value for
both fields, this means that the default for breakpoint::task has
changed from 0 to -1.   I've updated all the code I can find that
relied on the value of 0, and I see no test regressions, especially in
gdb.ada/tasks.exp, which still fully passes.

There should be no user visible changes after this commit.

Approved-By: Pedro Alves <pedro@palves.net>