ftoastr, snprintf: ftoastr + snprintf module

ftoastr, snprintf: ftoastr + snprintf module

* lib/ftoastr.c: Use GNULIB_SNPRINTF, not GNULIB_SNPRINTF_POSIX,
since the snprintf module now should be good enough here.
* modules/snprintf (configure.ac): Add gl_MODULE_INDICATOR([snprintf]).
It seems odd to have both gl_STDIO_MODULE_INDICATOR([snprintf])
and gl_MODULE_INDICATOR([snprintf]), but the former enables
GNULIB_SNPRINTF only for the test directory, and the latter
doesn't arrange for gl_STDIO_H_DEFAULTS to be called, so neither
seems to suffice by itself.