On MS-Windows, define _WIN32_WINNT in a single common place.

On MS-Windows, define _WIN32_WINNT in a single common place.

This changeset defines _WIN32_WINNT to at least 0x0501, the level
of Windows XP, unless defined to a higher level, in a single
place.  It then removes all the overrides of _WIN32_WINNT in
individual files as no longer needed.  Doing this also solves
compilation of windows-nat.c with mingw.org's MinGW, as that
file uses CONSOLE_FONT_INFO which needs the XP level to become
exposed in the Windows headers, while mingw.org defaults to
Windows 9X.

gdb/ChangeLog:
2019-05-03  Eli Zaretskii  <eliz@gnu.org>

	* common/common-defs.h [__MINGW32__ || __CYGWIN__]: Define
	_WIN32_WINNT to the XP level, unless already defined to a higher
	level.

	* unittests/parse-connection-spec-selftests.c:
	* ser-tcp.c:
	* common/netstuff.c [USE_WIN32API]:  Remove the _WIN32_WINNT
	override.

gdb/gdbserver/ChangeLog:
2019-05-03  Eli Zaretskii  <eliz@gnu.org>

	* remote-utils.c:
	* gdbreplay.c [USE_WIN32API]: Remove the _WIN32_WINNT override.