pw useradd: Validate the user name before creating the entry

Previouly it was possible to create users with spaces in the name with:
pw useradd -u 1234 -g 1234 -n 'test user'

The "-g 1234" is relevant, without it the name was already rejected
as expected:

[fk@test ~]$ sudo pw useradd -u 1234 -n 'test user'
pw: invalid character ` ' at position 4 in userid/group name

Bug unintentionally found with a salt config without explicit name entry:

test user:

user.present:
  - uid: 1234
  - gid: 1234
  - fullname: Test user
  - shell: /usr/local/bin/bash
  - home: /home/test
  - groups:
    - wheel
    - salt

"Luckily" salt modules rarely bother with input validation either ...

PR: 221416
Submitted by: Fabian Keil
Obtained from: ElectroBSD
MFC after: 1 week