src/newusers.c: A user/group name with a leading digit is valid

src/newusers.c: A user/group name with a leading digit is valid

Only consider a string to be a UID/GID if it is all digits.

Here's a reproducer of the bug:

	$ echo 'foo:p::1a::/tmp/nonexistent:/usr/bin/false' > x
	$ sudo newusers ./x
	newusers: invalid group ID '1a'
	newusers: line 1: can't create group

Where the expected behavior would be the same as for a group name that
doesn't start with a digit:

	$ echo 'foo:p::a1a::/tmp/nonexistent:/usr/bin/false' > x
	$ sudo newusers ./x
	$ tail -n1 /etc/group
	a1a:x:1004:
	$ tail -n1 /etc/passwd
	foo:x:1004:1004::/tmp/nonexistent:/usr/bin/false

Closes: <https://github.com/shadow-maint/shadow/issues/1474>
Signed-off-by: Alejandro Colomar <alx@kernel.org>