Hi,

This patch adds support to the remote protocol to disable packet
acknowlegment.  This is useful when the transport being used is itself
reliable, e.g., TCP/IP.  In these cases, by removing the acking
we reduce the amount of roundtrips, and decrease the communication
latency.

While we were discussing implementing this, I noticed that Apple
already had something of the sort, so we ended up reusing Apple's
QStartNoAckMode packet name, and basing our implementation on
their version, modernizing it, adding qSupport support
to it, tweaking the user interface so it is integrated with
the "set remote xxx-packet" set of commands, and documenting it.

The docs changes below explain more in detail the approach taken.

I'm including in the patch GDB changes to support the feature,
and adding the support to gdbserver.  Regtesting against a native
gdbserver on x86_64-unknown-linux-gnu doesn't show any regression.

What do you think?

-- 
Pedro Alves