journal: new logging macros to include UNIT=

journal: new logging macros to include UNIT=

Adding UNIT= to log lines allows them to be shown
in 'systemctl status' output, etc.

A new set of macros and functions is added. This allows for less
verbose notation than using log_struct() explicitly.

The set of logging functions is expanded to take a pair of arguments
(e.g. "UNIT=" and the RHS) which add an extra line to the structured
log entry. This can be used to add macros which add a different
identifier later on.