Windows socket error numbers are not reported using errno, but with
WSAGetLastError(). _getdns_errnostr() and friends as implemented on
Windows don't work for errors resulting from file open/close/read/write
etc.
So add a parallel set of functions specifically for file errors.
sys/poll.h seems to be some GNU extension. musl warns about this:
warning redirecting incorrect #include <sys/poll.h> to <poll.h>
Signed-off-by: Rosen Penev <rosenp@gmail.com>