luci-base: set password also for rpcd user

luci-base: set password also for rpcd user

This change allows the root user to change also rpcd logins via
administration tab in system. This changes `setPassword` in a way that
existing calls to it fall back to change unix passwords.

The goal of this commit is to prepare the environment to add password
policy (password length, valid property, special characters, etc)
to the administration tab.

Signed-off-by: Christian Korber <redacted>