RFC: callbacks for rsinit

[KarlK90]

This is a quick demonstrator on how to implement the functionality of #21 as callbacks. More work needs to be done to truly separate the custom logic from the main rsinit crate, but the idea should be clear :-).

This is the output of this rsinit implementation run inside qemu:

Running /init...
[    2.576598]  rsinit: Custom option provided: wowowowowow
[    2.578203]  rsinit: Beep, beep! Post setup callback running... I'll just print the options: CmdlineOptions {
[    2.578203]     root: Some(
[    2.578203]         "/dev/vdb",
[    2.578203]     ),
[    2.578203]     rootfstype: Some(
[    2.578203]         "ext4",
[    2.578203]     ),
[    2.578203]     rootflags: None,
[    2.578203]     rootfsflags: MsFlags(
[    2.578203]         0x0,
[    2.578203]     ),
[    2.578203]     nfsroot: None,
[    2.578203]     init: "/sbin/init",
[    2.578203]     cleanup: true,
[    2.578203]     callbacks: CmdlineOptionsCallbacks {
[    2.578203]         callbacks_count: 0,
[    2.578203]     },
[    2.578203] }
[    2.624939] EXT4-fs (vdb): recovery complete
[    2.626477] EXT4-fs (vdb): mounted filesystem 59e66880-dd9c-430a-af55-52986ed24ac7 r/w with ordered data mode. Quota mode: none.
[    2.627301]  rsinit: I'm lucky, I was the first callback to be added!
[    2.627569]  rsinit: Let me show you the bind mounts collected so far: ["/lib/modules"]
[    2.628059]  rsinit: Bind mounting /lib/modules to /root/lib/modules