Bug 20861

read(), write(), et al. in libc_r do not honor socket timeouts set via
setsockopt(sd, SOL_SOCKET, SO_{SND,RCV}TIMEO, ...). Timeout is always
set to NULL and the read loops consume EAGAIN.

Fix: 

In read/write/etc. implementation, do a S_ISSOCK() on the descriptor,
and if true, get a proper timeout value using _thread_sys_getsockopt()
instead of assuming NULL. When the timeout expires, return EAGAIN if
a proper timeout was used.
How-To-Repeat: s = socket(...);
setsockopt(s, SOL_SOCKET, SO_RCVTIMEO, (void*) &timeout, sizeof(timeout));
connect(s, ...);
read(s, buf, nbytes); <-- blocks and never returns EAGAIN