lib/: Use strisdigit() instead of its pattern

lib/: Use strisdigit() instead of its pattern

Note that the old code in

	(1)  lib/strtoday.c:strtoday()
	(2)  lib/subordinateio.c:append_uids()

was considering an empty string as if it were a number.
strisdigit() does not consider an empty string to be numeric.

I think it will not affect the behavior in either case, as they should
sooner or later result in an error somewhere.  And it seems (IMO)
surprising to treat empty strings as numeric strings, so let's not do
it.

Reviewed-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>