Improve user creation

Fixes #709 (closed)

Important note

Outdated instructions, these are not necessary anymore with the safe upgrading path, that was implemented later in this PR! Just confirm that you want to migrate your work folder, it should all go automatically.

After merging or changing branch for testing, you need to run the following commands to set your pmbootstrap folder in a consistent state. These are the steps I had to run after changing branch from master:

pmb init
pmb -y zap -d -hc -p
rm -fr ~/.local/var/pmbootstrap/config_*

Or you will find these kind of errors:

(010391) [22:31:44] (native) % cd /home/build/packages/build/aarch64 && su build -c 'abuild-sign APKINDEX.tar.gz_'
Error opening key file /home/user/.abuild/user-59da88ae.rsa
139767470046092:error:02FFF002:system library:func(4095):No such file or directory:bio/bss_file.c:255:fopen('/home/user/.abuild/user-59da88ae.rsa', 'r')
139767470046092:error:20FFF002:BIO routines:CRYPTO_internal:system lib:bio/bss_file.c:257:
unable to load key file

ERROR: /home/build/packages/build/x86_64/postmarketos-base-3-r8.apk: UNTRUSTED signature

@ollieparanoid Please remember to add the needed commands to the commit message.