gdbstub: Handle errors in gdb_accept()

gdbstub: Handle errors in gdb_accept()

In gdb_accept(), we both fail to check all errors (notably
that from socket_set_nodelay(), as Coverity notes in CID 1005666),
and fail to return an error status back to our caller. Correct
both of these things, so that errors in accept() result in our
stopping with a useful error message rather than ignoring it.

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Signed-off-by: Michael Tokarev <[email protected]>