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.