journald: only accept valid unit names for log streams

journald: only accept valid unit names for log streams

Let's be a bit stricter in what we end up logging: ignore invalid unit
name specifications. Let's validate all input!

As we ignore unit names passed in from unprivileged clients anyway the
effect of this additional check is minimal.

(Also, no need to initialize the identifier/unit_id fields of stream
objects to NULL if empty strings are passed, the default is NULL
anyway...)