ExpandString correctly loops on the syscall until it reaches the
required buffer size but truncates it before converting it back to
string. The truncation limit is increased to 2^15 bytes which is the
documented maximum ExpandEnvironmentStrings output size.

This fixes TestExpandString on systems where len($PATH) > 1024.

Change-Id: I2a6f184eeca939121b458bcffe1a436a50f3298e
Reviewed-on: https://go-review.googlesource.com/9805Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>