Previously useradd would not check if a username already existed on the
system, and would instead add the user anyway and just increment the
uid. useradd should instead return an error when the user attempts to
create already existing users.
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
