justify checking for socklen_t in more than one function, the code
used to find a valid socklen_t replacement is simplified back. The
only function that will be used to find a socklen_t replacement is
getpeername, as it has been since revision 1.4 of curl/acinclude.m4
Make UDP sockets non-blocking. I've confirmed that at least on Linux 2.4 a
read event can come back from poll() on a valid SOCK_DGRAM socket but
recv(2) will still block. This patch doesn't ignore EAGAIN in
read_udp_packets(), though maybe it should. (This patch was edited by Daniel
Stenberg and a new configure test was added (imported from curl's configure)
to properly detect what non-blocking socket approach to use.)
