Extract the users/groups creation into a separate shell script.

This adds a few features:

• it checks that the UID line has the right number of fields [1]
• it tells the user that there are groups that he may want to remove [2]

PR: 208800, 173318 [1], 157546 [2]
Submitted by: mat
Sponsored by: Absolight
Differential Revision: https://reviews.freebsd.org/D5939