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/core/main.c

@@ -497,8 +497,7 @@ static int config_parse_show_status(
 
         k = parse_show_status(rvalue, b);
         if (k < 0) {
-                log_syntax(unit, LOG_ERR, filename, line, -k,
-                           "Failed to parse show status setting, ignoring: %s", rvalue);
+                log_syntax(unit, LOG_ERR, filename, line, k, "Failed to parse show status setting, ignoring: %s", rvalue);
                 return 0;
         }
 
@@ -629,8 +628,7 @@ static int config_parse_join_controllers(const char *unit,
                 }
         }
         if (!isempty(rvalue))
-                log_syntax(unit, LOG_ERR, filename, line, EINVAL,
-                           "Trailing garbage, ignoring.");
+                log_syntax(unit, LOG_ERR, filename, line, 0, "Trailing garbage, ignoring.");
 
         return 0;
 }