logger: Define SD_JOURNAL_SUPPRESS_LOCATION.

logger: Define SD_JOURNAL_SUPPRESS_LOCATION.

The normal journald functions add the location in the C source code files to
the log messages. This is nice for a big C based project, but logger is used in
scripts so it would be more useful to let users specify the location in the
script by adding the CODE_FUNC, CODE_FILE and CODE_FILE fields to the log
message.

It is already possible to do this, but it will result in two versions of these
fields: one for the location in logger.c and one for the location in the
script.