Description of problem:
useradd allows digit only usernames. That, however, breaks some basic tools that take both username and UID as their argument, like setfacl or getent:

[root@rhel7 ~]# grep 500001 /etc/passwd
500001:x:1001:1001::/home/500001:/bin/bash
test:x:500001:1001::/tmp:/bin/bash
[root@rhel7 ~]# getfacl /tmp/test 
getfacl: Removing leading '/' from absolute path names
# file: tmp/test
# owner: root
# group: root
user::rw-
group::r--
other::r--

[root@rhel7 ~]# setfacl -m u:500001:r /tmp/test 
[root@rhel7 ~]# getfacl /tmp/test 
getfacl: Removing leading '/' from absolute path names
# file: tmp/test
# owner: root
# group: root
user::rw-
user:test:r--
group::r--
mask::r--
other::r--

[root@rhel7 ~]# getent passwd 500001
test:x:500001:1001::/tmp:/bin/bash
[root@rhel7 ~]# 

Version-Release number of selected component (if applicable):
shadow-utils-4.1.5.1-13.el7.x86_64

How reproducible:
always

Steps to Reproduce:
1. try to create a user with digits only username

Actual results:
The user gets created.

Expected results:
The user does not get created and appropriate error message is printed (useradd: invalid user name).

Additional info:
N/A