Previously, std.socket code assumed that MSG_NOSIGNAL existed on all platforms, which is not the case – std.c.osx.socket actually has a comment saying »Not defined in OS X, but we'll use them anyway«. This lead to Socket.send() raising SIGPIPE on OS X if the peer had already closed the connection.
With this commit, std.socket sets the SO_NOSIGPIPE socket option on systems that support it for the same effect as MSG_NOSIGNAL on Linux.
Strictly speaking, removing the NOSIGNAL SocketFlag is a breaking API change, but I do not think there is any code actually using it, as it has always been set implicitly by std.socket anyway.
* Transformed std.c.linux.* into wrappers around core.sys.posix modules where appropriate.
* Added std.c.osx as the OSX equivalent of std.c.linux.
* Added std.c.osx.socket for OSX-specific socket declarations not covered by the Posix spec.
* Altered all makefiles to zip, etc, the new std.c.osx package.
