shared/conf-parser: say "key name" not "lvalue", add dot

"lvalue" is our internal jargon. Let's try not to confuse non-programmers.

src/shared/conf-parser.c

@@ -138,7 +138,8 @@ static int next_assignment(
 
         /* Warn about unknown non-extension fields. */
         if (!(flags & CONFIG_PARSE_RELAXED) && !startswith(lvalue, "X-"))
-                log_syntax(unit, LOG_WARNING, filename, line, 0, "Unknown lvalue '%s' in section '%s', ignoring", lvalue, section);
+                log_syntax(unit, LOG_WARNING, filename, line, 0,
+                           "Unknown key name '%s' in section '%s', ignoring.", lvalue, section);
 
         return 0;
 }