gdb: use bool instead of int in struct internal_problem

gdb: use bool instead of int in struct internal_problem

Change struct internal_problem (gdb/utils.c) to use bool instead of
int, update the 3 static instances of this structure that we create to
use true/false instead of 1/0.

I've also updated the comments on struct internal_problem as the
existing comment doesn't seem to be referring to the structure, it
talks about returning something, which doesn't make sense in this
context.

There should be no user visible changes after this commit.