Switch from numerical to defined constants for permissions.

Switch from numerical to defined constants for permissions.

Use defined constants for permission values. Also add fallback
definitions for them in system.h, to allow for compatibility with
systems that don't provide these macros.

Include system.h in all tests/ files that required it.

Signed-off-by: Érico Rolim <erico.erc@gmail.com>
Signed-off-by: Mark Wielaard <mark@klomp.org>