This reverts commit fab50488fcb741884ccdfa7b83643eac3e5c9cbf.
The function was, on reflection, not important enough to break the feature
freeze, since it's trivial to build on your own.
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)