src: improve attributes

src: improve attributes

- Properly guard odhcp6c.h attributes with `#ifndef` to avoid redefining
them when including external headers.
- Also convert the remaining __attribute__ usages to the custom
declarations.
- Consolidate custom declarations with _o_ prefix.

Signed-off-by: Álvaro Fernández Rojas <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/307