log: introduce log_parse_environment_cli() and log_setup_cli()

Presently, CLI utilities such as systemctl will check whether they have a tty
attached or not to decide whether to parse /proc/cmdline or EFI variable
SystemdOptions looking for systemd.log_* entries.

But this check will be misleading if these tools are being launched by a
daemon, such as a monitoring daemon or automation service that runs in
background.

Make log handling of CLI tools uniform by never checking /proc/cmdline or EFI
variables to determine the logging level.

Furthermore, introduce a new log_setup_cli() shortcut to set up common options
used by most command-line utilities.