AC_SOCKET_INVALID nitpick: ((unsigned)~0) [needs 2's compl.] -> ((unsigned)-1)

be sent to transmit client credentials. Buffer the message data.

(with notable exceptions: ignore tests for EINTR which winsock never sets)

For ITS#4651

on Solaris, [v][sf][n]printf doesn't like NULL; on Solaris 10 strerror_r apparently returns NULL in some cases (ITS#4285)

Might need a worse #ifdef mess if any non-glibc memrchr() turns up.