vasnprintf: Support I flag on glibc systems.

* lib/printf-parse.h (FLAG_LOCALIZED): New macro.
* lib/printf-parse.c (PRINTF_PARSE): Handle the 'I' flag.
* lib/vasnprintf.c (VASNPRINTF): Pass the 'I' flag on to the system's
snprintf function.
* tests/test-vasnprintf-posix.c (test_function): Test the 'I' flag on
glibc systems.
* tests/test-vasnprintf-posix3.c: New file.
* modules/vasnprintf-posix-tests (Files): Add it.
(TESTS, check_PROGRAMS): Add test-vasnprintf-posix3.