Fix Linux dependencies and GNUisms #3
Description
Just for the fun of it, it would be cool to get the init running on non-Linux systems.
For things like signalfd, some kind of abstraction might be used with a self-pipe implementation on other systems.
Various system calls are probably used in ways that implicitly depend on Linux behaviour in corner cases.
Some GNUisms would also have to be cleaned up, but most of them have already been duplicated by other compilers and libc implementations.