Hide internal idna functions [BZ #18822]

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

	[BZ #18822]
	* include/idna.h: New file.
	* inet/getnameinfo.c: Include <idna.h> instead of
	<libidn/idna.h>.
	(__idna_to_unicode_lzlz): Removed.
	* sysdeps/posix/getaddrinfo.c: Include <idna.h> instead of
	<libidn/idna.h>.
	(__idna_to_ascii_lz): Removed.
	(__idna_to_unicode_lzlz): Likewise.