coio: handle spurious wakeup correctly

coio_accept, coio_read, coio_write, coio_writev used to handle spurious
wakeups correctly: if the timeout hasn't passed yet, they would simply
retry reading (or writing) and fall asleep once again if no data is
ready.

This behaviour changed in the following patches:
577a640a ("coio: pass fd to
coio_accept") and 4f84859d ("Introduce
iostream wrapper for socket I/O").

Now the functions timeout on the very first spurious wakeup.

Fix this and add the appropriate unit tests.