inf_ttrace_follow_fork: detach_breakpoints called twice for child.

inf_ttrace_follow_fork: detach_breakpoints called twice for child.

When debugging a program that forks with follow-fork set to follow
the parent, we end up calling detach_breakpoints for the child twice.
On ia64-hpux, this leads to a warning when trying to remove the
breakpoints the second time around, because the ia64 code detects
that the address does not point to a breakpoint instruction.

gdb/ChangeLog:

        * inf-ttrace.c (inf_ttrace_follow_fork): When following the
        parent, only call detach_breakpoints if tts.tts_event ==
        TTEVT_VFORK.