backup_control: Rework function interfaces
backup_control: Rework function interfaces

Change all relevant functions to return `UResult`s from `BackupError` instead
of error strings. Make `determine_backup_mode/suffix` accept `clap::ArgMatches`
as input argument to parse for the arguments themselves, using the arguments
with are defined in the `arguments` submodule.

This way the user only needs to include the pre-defined arguments from the
`arguments` module and passes a reference to the applications `ArgMatches` into
the respective functions here. The functions then take care of handling the
arguments. It is recommended to use the arguments provided in the `arguments`
module over custom-defined ones.

uutils/coreutils,uutils/coreutils,uutils/coreutils