Fix PR win32/24284: tcp_auto_retry doesn't work in MinGW

Fix PR win32/24284: tcp_auto_retry doesn't work in MinGW

This was reported by Bernhard Wodok, along with a patch to fix the
issue.  I adjusted the patch a bit, and I'm submitting the patch on
his behalf.

According to Bernhard, the issue can be reproduced by doing:

  1. start gdb
  2. enter 'target remote :2345'
  3. observe that it throws a "connection refused" error immediately
  instead of waiting and throwing a timeout error

I.e., I believe it can be reproduced by our current tests, which is
why I'm not proposing any extra tests here (well, I don't use nor have
any Windows system to test this, so...).

The problem happens because, on ser-tcp:wait_for_connect, we call
'gdb_select' passing 0 as its first argument, which, when using MinGW,
ends up using the 'gdb_select' version from mingw-hdep.c, and when the
first argument is 0 this means that WaitForMultipleObjects will be
called with 0 as its first argument as well.  According to the MS API
docs, this is forbidden:

  https://docs.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-waitformultipleobjects

The proposed fix is simple: we just call Sleep when N == 0 (and when
TIMEOUT is non-NULL), and return 0.  It makes sense to me.

Both Bernhard and Paul Carroll confirmed that the fix works.  I'm
Cc'ing Bernhard in case you have any questions about the patch.

OK?

gdb/ChangeLog:
2019-08-29  Bernhard Wodok  <barto@gmx.net>
	    Sergio Durigan Junior  <sergiodj@redhat.com>

	PR win32/24284
	* mingw-hdep.c (gdb_select): Handle case when 'n' is zero.