popen: use of exception safe functions for IO

popen read / write functions are written in C and intended to be used
from C: the contract is to return -1 at failure and set an entry to the
diagnostics area. However a C++ exception from coio read / write
functions would pass over a popen function stack frame.

The solution is to use the recently introduced coio exception safe
functions.

Part of #4031

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