Adds systemd_namespace_sink that can open a stream to a specific systemd
journal namespace for logging. This was created as a separate sink from
systemd_sink because it has significant difference due to systemd's
journal API: `sd_journal_send()` does not have namespace-specific
counterpart and thus this cannot utilize other extra journal fields than
`PRIORITY` (via kernel-style log level prefixes).

Another difference is that this API is only available from systemd
version 256, thus a version check was added to CMake file for the unit
tests so that they are not automatically included if `libsystemd` is not
new enough.

Unit tests are split to two parts: first one runs always and it just
uses the stream to log to default namespace (should always work). Second
one requires manual setting up of "custom" journal namespace and thus is
ignored from the default set of tests (can be run by specifying
`[systemd_namespace]` tag explicitly when running tests). On the other
hand, the second part also automatically checks the output correctness
from journal.

`SD_JOURNAL_SUPPRESS_LOCATION` is not needed by this sink since it does
not use the APIs that add the location info.

Moved level prefixes to `static constexpr` to indicate those being
compile-time constants (and also saves a bit space).

Removed message length truncation since with this stream API we don't
even know the maximum length of the messages. Journald truncates the
message at its end based on its own configuration.

Added error checking to `fwrite_bytes()` calls similarly to other sinks.

Optimized away excess `\n` check.

Added `flush_()` implementation.