Improved "user" creation code

Created by: ollieparanoid

Follow-up to #666 (closed) and what has been discussed in the channel. We would like to have the following changes:

• user used in the build chroots should be called "build", not "user" (to avoid confusion)
• ask in pmbootstrap init for the install-user, default to user
• during pmbootstrap install: the install-user should be created inside the chroot, with a UID of 1000
• give the install-user the normal set of "desktop" groups

Keep in mind, that a lot of --bind mounts mount stuff to /home/user, so we would need to replace the username in there.