Hi,

A couple of weeks ago, Dmitry Smirnov reported that GDB was trying to get
qThreadExtraInfo out of his target and it was damaging his debug session, as 
GDB complained about a protocol error -- because the reply that came back
was ill-formed.

If the remote side doesn't have any thread support, there's no use in
querying info about the main thread that GDB added internally.  One would
expect stubs that don't support threads to return empty to this optional
packet, but his report showed that it's better to not rely on
stubs behaving.

Tested against gdbserver with and without --disable-packet=threads.

OK?

-- 
Pedro Alves