core,network: major per-object logging rework

core,network: major per-object logging rework

This changes log_unit_info() (and friends) to take a real Unit* object
insted of just a unit name as parameter. The call will now prefix all
logged messages with the unit name, thus allowing the unit name to be
dropped from the various passed romat strings, simplifying invocations
drastically, and unifying log output across messages. Also, UNIT= vs.
USER_UNIT= is now derived from the Manager object attached to the Unit
object, instead of getpid(). This has the benefit of correcting the
field for --test runs.

Also contains a couple of other logging improvements:

- Drops a couple of strerror() invocations in favour of using %m.

- Not only .mount units now warn if a symlinks exist for the mount
  point already, .automount units do that too, now.

- A few invocations of log_struct() that didn't actually pass any
  additional structured data have been replaced by simpler invocations
  of log_unit_info() and friends.

- For structured data a new LOG_UNIT_MESSAGE() macro has been added,
  that works like LOG_MESSAGE() but prefixes the message with the unit
  name. Similar, there's now LOG_LINK_MESSAGE() and
  LOG_NETDEV_MESSAGE().

- For structured data new LOG_UNIT_ID(), LOG_LINK_INTERFACE(),
  LOG_NETDEV_INTERFACE() macros have been added that generate the
  necessary per object fields. The old log_unit_struct() call has been
  removed in favour of these new macros used in raw log_struct()
  invocations. In addition to removing one more function call this
  allows generated structured log messages that contain two object
  fields, as necessary for example for network interfaces that are
  joined into another network interface, and whose messages shall be
  indexed by both.

- The LOG_ERRNO() macro has been removed, in favour of
  log_struct_errno(). The latter has the benefit of ensuring that %m in
  format strings is properly resolved to the specified error number.

- A number of logging messages have been converted to use
  log_unit_info() instead of log_info()

- The client code in sysv-generator no longer #includes core code from
  src/core/.

- log_unit_full_errno() has been removed, log_unit_full() instead takes
  an errno now, too.

- log_unit_info(), log_link_info(), log_netdev_info() and friends, now
  avoid double evaluation of their parameters