Rename double underscore macros:
* rename __unused to _unused
* rename __maybe_unused to _maybe_used
* rename __not_aligned to _not_aligned

Ubuntu 12.04 fails to compile because of a variable called __unused
inside struct stat. The problem didn't happen before because we were
defining __unused after including sys/stat.h.
GNU says that identifiers with a double underscore are reserved.
Hopefully using a single underscore will be enough.