[Issue #5] Implemented WinSock variation of "ser_drain(...)" functionality

[ccooper21]

This pull request corresponds to avrdude patch #9304. I'm not sure how patch numbering and ordering is determined, so I used 99. I imagine this should be changed, but the ordering is largely immaterial since the file changed by this patch, ser_win32.c, is not changed by any of the other patches that presently exist.

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