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/shared/install.c

@@ -949,8 +949,7 @@ static int config_parse_also(
                         return r;
         }
         if (!isempty(state))
-                log_syntax(unit, LOG_ERR, filename, line, EINVAL,
-                           "Trailing garbage, ignoring.");
+                log_syntax(unit, LOG_ERR, filename, line, 0, "Trailing garbage, ignoring.");
 
         return 0;
 }