tree-wide: clean up log_syntax() usage

- Rely everywhere that we use abs() on the error code passed in anyway,
  thus don't need to explicitly negate what we pass in

- Never attach synthetic error number information to log messages. Only
  log about errors we *receive* with the error number we got there,
  don't log any synthetic error, that don#t even propagate, but just eat
  up.

- Be more careful with attaching exactly the error we get, instead of
  errno or unrelated errors randomly.

- Fix one occasion where the error number and line number got swapped.

- Make sure we never tape over OOM issues, or inability to resolve
  specifiers

src/nspawn/nspawn-settings.c

@@ -152,7 +152,7 @@ int config_parse_capability(
 
                 cap = capability_from_name(word);
                 if (cap < 0) {
-                        log_syntax(unit, LOG_ERR, filename, line, cap, "Failed to parse capability, ignoring: %s", word);
+                        log_syntax(unit, LOG_ERR, filename, line, 0, "Failed to parse capability, ignoring: %s", word);
                         continue;
                 }