copy attach_flag to fork children

[Pedro Alves]

This restores the behaviour gdb had when attach_flag was a global, instead
of beeing an inferior property.  This way, both terminal_inferior/terminal_ours
will behave identically in fork children, and, gdb will default to want to
detach from forked children of attached inferiors too.

While testing on OpenBSD, I noticed that the detach_breakpoints call
was asserting, because it was now being done before updating inferior_ptid.

Fixed that, did a testrun on x86_64-linux, and checked in.

-- 
Pedro Alves
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