mbox does not support i686
update-users-groups.pl: Generate hashed passwords internally

I.e. don't call "passwd" to update /etc/shadow from the "password"
option. This has the side-effect of not updating the password if
mutableUsers = true (since the code path for "hashedPassword" has a
check for mutableUsers).

Fixes #4747.
Merge pull request #4755 from ehmry/mbox

mbox does not support i686
postgresql 9.4beta3, but keep beta2 around because they are not compatible
nixos/nat: Fix override so that sysctls are properly preserved