popen: unblock popen_read_timeout at a first byte

Before this change popen_read_timeout() waits until a passed buffer will
be fully filled (or until EOF / timeout / IO error occurs). Now it waits
for any amount of data (but at least one byte).

It allows to communicate with an interactive child program: write, read,
repeat.

Part of #4031

Acked-by: Cyrill Gorcunov <gorcunov@gmail.com>