journal: implicitly add code location to all messages logged with the native interface

      This logic can be turned off by defining SD_JOURNAL_SUPPRESS_LOCATION
before including sd-journal.h.

This also saves/restores errno in all logging functions, in order to be
useful as logging calls without side-effects.

This also adds a couple of __unlikely__ around the early checks in the
logging calls, in order to minimize the runtime impact.