gdbstub: avoid busy loop while waiting for gdb

While waiting for a gdb response, or while sending an acknowledgement
there is not much to do, so do not mark the socket as non-blocking to
avoid a busy loop while paused at gdb. This only affects the user-mode
emulation (qemu-arm -g 1234 ./a.out).

Note that this issue was reported before at
https://lists.nongnu.org/archive/html/qemu-devel/2013-02/msg02277.html.

While at it, close the gdb client fd on EOF or error while reading.

Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
