utils: Replace unsafe code with Nix new User and Group methods
utils: Replace unsafe code with Nix new User and Group methods

The Nix crate now allow to find the User and Group numeric id from its
name. This internally still uses unsafe code but it goes a long way to
avoid undefined behavior so we better use it.

Signed-off-by: Otavio Salvador <1d31b11c1b92bf5d043d36edfe7174db39a6b891@ossystems.com.br>

UpdateHub/updatehub