popen: fix popen_write_timeout retval type

On Linux x86_64 `ssize_t` is 64 bit, while `int` is 32 bit wide (at
least typically). Let's return `ssize_t` from popen_write_timeout() to
prevent data loss.

Part of #4031

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