vsnprintf: make more consistent with snprintf; doc fixes

vsnprintf: make more consistent with snprintf; doc fixes

* doc/posix-functions/snprintf.texi (snprintf): The workaround for
the byte count return problem was promoted from the snprintf-posix
to the snprintf module.
* doc/posix-functions/vsnprintf.texi (vsnprintf): Likewise.
* m4/vsnprintf.m4 (gl_FUNC_VSNPRINTF): Also check
gl_SNPRINTF_RETVAL_C99, for consistency with gl_FUNC_SNPRINTF.
* tests/test-snprintf.c (main): Check the byte count returned.
* tests/test-vsnprintf.c (main): Likewise.