Close #709: Improve user creation (#725)

* Allow to specify a custom username in "pmbootstrap init"
* Build chroots have "pmos" instead of "user" as username now
* Installation user UID is 1000 now (as in all other Linux distributions)
* Adjust autologins
* postmarketos-base: enable wheel group for sudo, removed previous sudoers file
* Implement safe upgrade path:
We save the version of the work folder format now, in $WORK/version.
When this file does not exist, it defaults to 0.
In case it does not match the currently required version
(pmb.config.work_version), then ask the user if it should
automatically be upgraded.