Mark internal argz functions with attribute_hidden [BZ #18822]

Move internal argz function prototypes to include/argz.h and mark them
with attribute_hidden to allow direct access within libc.so and libc.a
without using GOT nor PLT.  This also brings string/argz.h closer to the
gnulib version.

	[BZ #18822]
	* include/argz.h (__argz_create_sep): New function prototype.
	(__argz_append): Likewise.
	(__argz_add): Likewise.
	(__argz_add_sep): Likewise.
	(__argz_delete): Likewise.
	(__argz_insert): Likewise.
	(__argz_replace): Likewise.
	* string/argz.h (__argz_create_sep): Removed.
	(__argz_append): Likewise.
	(__argz_add): Likewise.
	(__argz_add_sep): Likewise.
	(__argz_delete): Likewise.
	(__argz_insert): Likewise.
	(__argz_replace): Likewise.