* fhandler.h (fhandler_socket::accept4): Rename from accept. Take
additional flag parameter.
* fhandler_socket.cc (fhandler_socket::accept4): Ditto. Handle
SOCK_NONBLOCK and SOCK_CLOEXEC flags.
* net.cc (cygwin_socket): Handle SOCK_NONBLOCK and SOCK_CLOEXEC flags
in type. Check for invalid flag values.
(socketpair): Ditto.
(cygwin_accept): Accommodate renaming of fhandler_socket::accept
function to accept4.
(accept4): New function.
* posix.sgml: Mention accept4 as GNU extensions.
* include/cygwin/socket.h (SOCK_NONBLOCK): Define.
(SOCK_CLOEXEC): Define.
(_SOCK_FLAG_MASK): Define when building Cygwin.
* include/cygwin/version.h (CYGWIN_VERSION_API_MINOR): Bump.
* include/sys/socket.h (accept4): Declare.
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