stdio: better 'inline'

stdio: better 'inline'

* lib/stdio.c: New file.
* lib/stdio.in.h (_GL_STDIO_INLINE):
New macro.  Replace all uses of 'static inline' with it.
Use _GL_INLINE_HEADER_BEGIN, _GL_INLINE_HEADER_END.
* m4/stdio_h.m4 (gl_STDIO_H): Do not require AC_C_INLINE.
* modules/stdio (Files, lib_SOURCES): Add lib/stdio.c.
(Depends-on): Add extern-inline.