journald: move all config entries from Manager to JournalConfig

journald: move all config entries from Manager to JournalConfig

Previously, only config entries controlled by multiple sources were located in
JournalConfig, and still other config entries were in Manager.
That's hard to maintain. Let's move all config entries to JournalConfig.

This also makes JournalConfig.forward_to_kmsg and friends tristate.
Otherwise, even if a higher precedence config source disables the
feature, it may be enabled by a lower precedence config.