This patch makes sure that the thread/task we attach to with ptrace
targets is added to GDB's thread table.  Targets that extend
inf-ptrace (inf_ptrace_target) should then use thread_change_ptid
as soon as then have more lwp or tid info, instead of just setting
inferior_ptid.  Targets that don't have a process_stratum target
overriding inf_ptrace_attach get it for free (BSD targets).

This was tested together with the rest of the series on
x86_64-unknown-linux-gnu, i386-unknown-openbsd4.3, i386-unknown-freebsd6.0
and i386-unknown-freebsd7.0.

OK?

-- 
Pedro Alves