Hide internal __ifreq function [BZ #18822]

Hide internal __ifreq function to allow direct access within libc.so and
libc.a without using GOT nor PLT.

	[BZ #18822]
	* include/ifreq.h: New file.
	* sysdeps/generic/ifreq.h (__if_nextreq): Removed.
	(__ifreq): Likewise.
	* sysdeps/mach/hurd/ifreq.h (__if_nextreq): Removed.
	(__ifreq): Likewise.