headers: Get glibc <libio.h> out of the default include path.

It's needed by GCC2 stdc++, but we shouldn't be including it
for all consumers of <stdio.h>, which we were. Drop it,
and adjust all the things that were relying on <stdio_pre.h>'s
definition of __PRINTFLIKE to use the _PRINTFLIKE defined
in BeBuild.h instead.

Hopefully should not cause any breakage; the libroot
implementation of __freading depended on this, but
I didn't see anything else which did.

headers/compatibility/bsd/stdio.h

@@ -19,7 +19,7 @@ extern "C" {
 
 char *fgetln(FILE *stream, size_t *_length);
 
-int asprintf(char **ret, char const *format, ...) __PRINTFLIKE(2,3);
+int asprintf(char **ret, char const *format, ...) _PRINTFLIKE(2,3);
 int vasprintf(char **ret, char const *format, va_list ap);