Fix: optional.h macro missing parentheses and guards

The are a few coding style issues with optional.h which leads to
unexpected effects when the macros are used in the caller code.

All macro parameters need to be surrounded by () (except when used near
commas, which is the C operator with least precedence).

All macros that emit code need to be surrounded by do { } while (0) so
not to emit extra ; or omit ;, which can cause subtle issues when used
with if/else statements.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Iba6fde7c267f4d8c9ec1a89147045f0bcda3a67a
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>