Replace EVUTIL_CLOSESOCKET macro with a function

Replace EVUTIL_CLOSESOCKET macro with a function

The EVUTIL_CLOSESOCKET() macro required you to include unistd.h in your
source for POSIX.  We might as well turn it into a function: an extra
function call is going to be cheap in comparison with the system call.

We retain the EVUTIL_CLOSESOCKET() macro as an alias for the new
evutil_closesocket() function.

(commit message from email by Nick and Sebastian)