Logging initialization was infallible previously, i.e., a logger would
always have to be chosen, falling back to our own logger
implementation in case of failure with setting up journald submission.
This lead to complexity like attempting to log errors from parsing
arguments, as there is a circular dependency.
Change this to make logging initialization fallible. If it fails, exit
non-zero instead of falling back to another logger. This makes the
initialization code slightly simpler.
At the same time, make choosing the logger possible via the command
line and deprecate `--log`.
Note that we still default to journald if detected.
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