Group ownership controlled by setgid bit. Refs #2492

Removed chown() calls from esmith DB libraries. Don't touch file
permissions, use defaults from filesystem: write access to root only,
read access to "adm" group members.

DB files are created through sudo calls and inherit default root umask
0022.
Fallback to root login if "admin" account is not available. Refs #2492

If admin passwd entry is missing, check "root" login password.

This hack gives the impression that admin user is available even if
nethserver-directory is not installed.
esmith::util (setUnixSystemPassword): check if admin account exists. Refs #2492

Also, honour AdminIsNotRoot key value (Refs #2277).