Feature test macros overhaul: stdio.h

Feature test macros overhaul: stdio.h

Throughout, remove references to __STRICT_ANSI__ and use the proper
internal macros and versions for C99, POSIX, ATFILE for the various *at
functions, or LARGEFILE for fseeko and ftello.

[v]asprintf are GNU extensions, but the *iprintf, *iscanf, and
*asnprintf functions are unique to newlib.

getw and putw were removed from POSIX.1-2001. funopen is BSD, and
fopencookie is GNU.

Signed-off-by: Yaakov Selkowitz <yselkowi@redhat.com>