motd: maintain a /etc/motd file with interesting info about the TAC

[KarlK90]

This is a refactor of #57

Not anyone looks at the web interface or the display of the TAC. This gives us a third vector to convey information to the user, by showing messages on ssh login.

Use bind-mounts to put the highly volatile file itself to /var/run and point to it from /etc.

[hnez]

I've had a look at the changes between #57 and this PR:

• Do not remove the /etc/motd bind mount when the motd::Motd struct is dropped. This simplifies the main.rs (but does mean that we may leave stale information behind. Which is okay though, as it is assumed that there will always be a running tacd).
• Use impl Display for Motd to format the message instead of a fully-custom method, which is a nice.
• Inline the color -codes into the writeln!() calls. This makes the lines quite long, but that's okay. (Line length was the reason I did not inline it initially).
• Use futures::select!() to watch all information sources instead of a lot of tasks and an ad-hoc queue. I do not like the readability and debuggability of macros, but this solution is more compact than the one in motd: maintain a /etc/motd file with interesting info about the TAC #57, which was not very elegant.
• Use MsFlags::MS_REMOUNT instead of explicit unmount + mount. Neat!

[hnez]

I will not get to the review of the changes now, but I think this PR is something we will want in the next meta-lxatac / tacos stable release and I will add a corresponding milestone.

[hnez]

I finally got around to having a look at this.
Looks good, thanks!