Fix thinko when deciding whether to disable TCP's Nagle algorithm

Fix thinko when deciding whether to disable TCP's Nagle algorithm

This patch fixes a thinko that happened when I was implementing the
IPv6 support on GDB/gdbserver.  On certain situations, it is necessary
to disable TCP's Nagle algorithm (NODELAY).  For obvious reasons, this
only applies when we are dealing with a TCP connection.

While implementing the IPv6 patch, I noticed that the net_open
function (on gdb/ser-tcp.c) kept a flag indicating whether the
connection type was UDP or TCP.  I eliminated that flag, and started
using the 'struct addrinfo *' related to the successful connection
directly.  However, I made a mistake:

  if (success_ainfo->ai_socktype == IPPROTO_TCP)
                     ^^^^^^^^^^^
    {
      /* Disable Nagle algorithm.  Needed in some cases.  */
      int tmp = 1;

      setsockopt (scb->fd, IPPROTO_TCP, TCP_NODELAY,
		  (char *) &tmp, sizeof (tmp));
    }

The 'ai_socktype' field specifies the socket type (SOCK_STREAM or
SOCK_DGRAM), and not the protocol.  This test was always failing, and
the Nagle algorithm was never being disabled.

The obvious fix is to use the 'ai_protocol' field.  This is what this
patch does.

Huge "thank you" to Joel Brobecker who reported the regression (he was
experiencing an unusual delay while debugging a bare-metal program
running under QEMU) and helped me set up a proper reproducer for the
bug.

gdb/ChangeLog:
2018-08-03  Sergio Durigan Junior  <sergiodj@redhat.com>

	* ser-tcp.c (net_open): Fix thinko when deciding whether to
	disable TCP's Nagle algorithm (use "ai_protocol" instead of
	"ai_socktype").