network: Don't redefine error codes if they already exist in errno.h

network: Don't redefine error codes if they already exist in errno.h

Since the errno.h values don't match the error codes that winsock
returns, map the winsock error codes to the errno ones, to make
sure explicit checks against AVERROR(x) match.

Signed-off-by: Martin Storsjö <martin@martin.st>