I removed the socklen_t use from the public curl/curl.h header and instead

I removed the socklen_t use from the public curl/curl.h header and instead
made it an unsigned int. The type was only used in the curl_sockaddr struct
definition (only used by the curl_opensocket_callback). On all platforms I
could find information about, socklen_t is 32 unsigned bits large so I don't
think this will break the API or ABI. The main reason for this change is of
course for all the platforms that don't have a socklen_t definition in their
headers to build fine again. Providing our own configure magic and custom
definition of socklen_t on those systems proved to work but was a lot of
cruft, code and extra magic needed - when this very small change of type seems
harmless and still solves the missing socklen_t problem.