terminal: extend RGB attributes

There're 3 supported color-modes: term-color-codes, 256-color-code and
rgb-color. We now use the term-color as default so zero(attr) will do what
you'd expect. Furthermore, we split rgb and 256color so users can forward
them properly without requiring an internal RGB converter.

Furthermore, a "hidden" field according to VT510rm manual is added.
terminal: add page handling for terminals

The page-layer is a one-dimensional array of lines. Combined with the
one-dimensional lines, you get a two-dimensional page. However, both
implementations, lines and pages only deal with their own dimension. That
means, lines don't know anything about other lines, and pages don't know
anything about cells.

Apart from pages, this also introduces history objects. A history object
is a scroll-back buffer. As some pages like alt-buffers don't have
histories, we keep them separate.

Pages itself forward all cell-related operations to the related line. Only
line-related operations are directly handled by the page. This is mostly
scrolling and history. To support proper resizing, we also keep a
fill-state just like lines do for cells.
terminal: add parser state-machine

The term-parser is used to parse any input from TTY-clients. It reads CSI,
DCS, OSC and ST control sequences and normal escape sequences. It doesn't
do anything with the parsed data besides detecting the sequence and
returning it. The caller has to react to them.

The parser also comes with its own UTF-8 helpers. The reason for that is
that we don't want to assert() or hard-fail on parsing errors. Instead,
we treat any invalid UTF-8 sequences as ISO-8859-1. This allows pasting
invalid data into a terminal (which cannot be controlled through the TTY,
anyway) and we still deal with it in a proper manner.
This is _required_ for 8-bit and 7-bit DEC modes (including the g0-g3
mappings), so it's not just an ugly fallback because we can (it's still
horribly ugly but at least we have an excuse).
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.
terminal: add systemd-subterm example

The systemd-subterm example is a stacked terminal that shows how to
use sd-term. Instead of rendering images and displaying it via X11/etc.,
it uses its parent terminal to display the page (terminal-emulator inside
a terminal-emulator) (like GNU-screen and friends do).

This is only for testing and not installed system-wide!
journal: reduce test-journal-send timeout from 10s to 1s

The sleep(10) in test-journal-send is quite aggressive. We need it only
for the journal to get our cgroup information. But even that information
is not vital to the test, so a sleep(1) should be just fine.
autogen: add "t" switch with --enable-terminal

Just temporarily add a "t" switch to "./autogen t" runs with
--enable-terminal. Once it's compile-tested enough, we can add it to the
default flags.