* net.cc (set_socket_inheritance): Rename from duplicate_socket. Use NT

* net.cc (set_socket_inheritance): Rename from duplicate_socket.  Use NT
specific call when appropriate.
(__set_winsock_errno): Rename from set_winsock_errno.  Accept function and line
as arguments.
* path.cc (path_conv::check): Continue the neverending battle to make cygwin
properly understand that path specs ending in a slash require that the path
refer to a directory.  Windows does not do this by default.
(symlink_info::check): Remove ENOTDIR test.  Move it to path_conv::check.
* path.h: Remove obsolete constant.
* winsup.h (set_winsock_errno): New macro.