Proposal: add System.IO.hGetBufSome

[Simon Marlow]

http://hackage.haskell.org/trac/ghc/ticket/4046

I'd like to add the following function to `System.IO`, alongside 
`hGetBuf` and `hGetBufNonBlocking`:

{{{
-- | 'hGetBufSome' @hdl buf count@ reads data from the handle @hdl@
-- into the buffer @buf at .  If there is any data available to read,
-- then 'hGetBufSome' returns it immediately; it only blocks if there
-- is no data to be read.
--
-- It returns the number of bytes actually read.  This may be zero if
-- EOF was reached before any data was read (or if @count@ is zero).
--
-- 'hGetBufSome' never raises an EOF exception, instead it returns a value
-- smaller than @count at .
--
-- If the handle is a pipe or socket, and the writing end
-- is closed, 'hGetBufSome' will behave as if EOF was reached.
--
-- 'hGetBufSome' ignores the prevailing 'TextEncoding' and 'NewlineMode'
-- on the 'Handle', and reads bytes directly.
}}}

We need it to properly fix #4041 and #3808.  It corresponds more closely 
to the semantics of Unix `read()`, and I've found this to be the most 
useful low-level API for reading in practice.

In the long term we'd like to provide a better binary I/O API, and we 
might even get around to doing that before 6.14.1, but if not we'll need 
this.

Deadline: 2 weeks.

Cheers,
	Simon