terminal: add screen-handling

terminal: add screen-handling

The screen-layer represents the terminal-side (compared to the host-side).
It connects term_parser with term_page and implements all the required
control sequences.

We do not implement all available control sequences. Even though our
parser recognizes them, there is no need to handle them. Most of them are
legacy or unused. We try to be as compatible to xterm, so if we missed
something, we can implement it later. However, all the VT510 / VT440 stuff
can safely be skipped (who needs terminal macros? WTF?).

The keyboard-handling is still missing. It will be added once
systemd-console is available and we pulled in the key-definitions.