Reading 4K at a time from a serial port turns out to be a bit

Reading 4K at a time from a serial port turns out to be a bit
unfriendly in an interactive session, because at 19200 baud it takes
nearly two seconds to receive that much data, and as long as the
data is flowing continuously Windows waits until it has a full
buffer. So here's another annoying flag in the winhandl API, which
restricts reads to length 1 so that serial output shows up as it
appears.

(I tried this yesterday, but without the OVERLAPPED fix in r6826 it
behaved very erratically. It now seems solid.)

git-svn-id: http://svn.tartarus.org/sgt/putty@6827 cda61777-01e9-0310-a592-d414129be87e