* sysdeps/mach/hurd/if_index.c: New file.

* sysdeps/mach/hurd/if_index.c: New file.

	* sysdeps/unix/sysv/linux/net/if.h: Moved to ...
	* sysdeps/gnu/net/if.h: here.
	(_IOT_ifconf, _IOT_ifreq): Hurd ioctl type macros moved from ...
	* sysdeps/mach/hurd/net/if.h: ... here; this file now removed.
	* sysdeps/generic/net/if.h: New file.  Declares just the standard
	functions and `struct if_nameindex' type.
	* socket/Makefile (headers): Add net/if.h here.
	* sysdeps/unix/sysv/linux/Makefile (sysdep_headers): Not here.
	* sysdeps/mach/hurd/Dist: Remove it here.
	* sysdeps/unix/sysv/linux/Dist: And here.
	* sysdeps/generic/if_index.c: Include <net/if.h>.