Mark internal argp functions with attribute_hidden [BZ #18822]

Mark internal argp functions with attribute_hidden [BZ #18822]

Mark internal argp functions with attribute_hidden to allow direct
access to them within libc.so and libc.a without using GOT nor PLT.

	[BZ #18822]
	* argp/argp-fmtstream.c: Include <argp-fmtstream.h>.
	* argp/argp-fs-xinl.c: Likewise.
	* argp/argp-help.c: Include <argp.h> and <argp-fmtstream.h>.
	* argp/argp-parse.c: Include <argp.h>.
	* argp/argp-xinl.c: Likewise.
	* include/argp-fmtstream.h: New file.
	* include/argp.h (__argp_error): Add attribute_hidden.
	(__argp_failure): Likewise.
	(__argp_input): Likewise.
	(__argp_state_help): Likewise.