networkd: improve how networkd logs things

networkd: improve how networkd logs things

This makes adds a couple of fixes:

- Introduces log_netdev_error_errno() and friends, which takes an error
  number, and matches what log_link_error_errno() and friends do.

- Replaces a lof ot strerror() usage with log_netdev_error_errno(),
  log_link_error_errno() and log_erro_errno()

- Uppercases the first character of many log messages, after all this is
  supposed to be english language

- Drops manual negating of error codes before passing them to log
  functions, the log functions all do that internally anyway.

Some other minor fixes.

Behaviour should not change really.