terminal: fix back-buffer selection on DRM page-flip

We currently select front-buffers as new back-buffer if they happen to be
the last buffer in our framebuffer-array. Fix this by never selecting a
new front buffer as back buffer.
terminal: make utf8 decoder return length

Lets return the parsed length in term_utf8_decode() instead of a buffer
pointer. Store the pointer in the passed argument.

This makes it adhere to the systemd coding-style, were we always avoid
returning pointers, but store them in output arguments. In this case, the
storage is not allocated, so it doesn't fit 100% to this idiom, but still
looks much nicer.
terminal/grdev: simplify DRM event parsing

Coverity complained about this code and is partially right. We are not
really protected against integer overflows. Sure, unlikely, but lets just
avoid any overflows and properly protect our parser loop.
terminal/drm: provide pipe->target() callback

Instead of looking for available back-buffers on each operation, set it to
NULL and wait for the next frame request. It will call back into the pipe
to request the back-buffer via ->target(), where we can do the same and
look for an available backbuffer.

This simplifies the code and avoids double lookups if we run short of
buffers.
terminal/grdev: provide front and back buffer to renderers

We really want more sophisticated aging than just 64bit integers. So
always provide front *and* back buffers to renderers so they can compare
arbitrary aging information and decide whether to re-render.
terminal/grdev: allow arbitrary fb-age contexts

Instead of limiting fb-aging to 64bit integers, allow any arbitrary
context together with a release function to free it once the FB is
destroyed.
terminal/drm: clear 'applied' flag when changing state

If a pipe is enabled/disabled, we have to clear crtc->applied of the
linked CRTC. Otherwise, we will not run a deep modeset, but leave the crtc
in the pre-configured state.
terminal: add screen renderer

We don't want to expose the term_screen internals for rendering.
Therefore, provide an iterator that allows external renderers to draw
terminals.
terminal/subterm: use screen renderer

Don't hard-code the screen renderer but use the newly introduced
term_screen_draw() helper.
terminal/unifont: add built-in fallback glyph

In case we cannot render a glyph, we want a fallback we can display
instead. If we rely on the font itself to provide the fallback character,
we have nothing to display if that character is not available. Therefore,
add a static fallback that we can use at any time.
terminal/idev: don't map XKB_KEY_NoSymbol as ASCII 0

XKB_KEY_NoSymbol is defined as 0 but does not correspond to a VT key with
ASCII value 0. No such key exists, so don't try to find such a key.
terminal/screen: add keyboard mapping

Implement the feed_keyboard() handling by mapping XKB keys according to
DEC-VT behavior.

Public information on terminal key-mappings is pretty scarce. We only
implement the most basic mapping for now. Further improvements welcome!
terminal/idev: add helper to match keyboard shortcuts

Matching keyboard shortcuts on internationalized keyboards is actually
non-trivial. Matching the actual key is easy, but the modifiers can be
used by both, the matching and the translation step. Therefore, XKB
exports "consumed-modifiers" that we use to figure out whether a modifier
was already used by the translation step.

The new IDEV_KBDMATCH() helper can be used to match on any keyboard
shortcut and it will do the right thing.
terminal/screen: mark cursor dirty on enabled/disable

If we hide or show the cursor, we change visual attributes and have to
mark the underlying cell as dirty. Otherwise, the terminal will not be
redrawn.
terminal/screen: add cursor rendering

This is the most simple way to render cursors: flip attr->inverse of the
cursor cell. This causes the background and foreground colors of the
cursor-cell to be inversed.

Now that we render cursors ourselves, make subterm not call into the
parent terminal to render cursors.
terminal/screen: add color converter

Terminals use pseudo color-codes mixed with 8bit and 24bit colors. Provide
a color-converter so external renderers only have to deal with ARGB32
colors.

This requires a color-palette as input as there's no fixed mapping. We
provide a default, but maybe we wanna support external palettes in the
future.
terminal/screen: adjust screen age only on update

Instead of increasing the screen-age on redraw, we now increase it only on
real updates. This is effectively the same, but avoids increased age
counters on backbuffer rendering. Therefore, we can now check age counters
against fronbuffers safely, while rendering frames in background.
pty: optimize read loop

As it turns out, I can actually send data to the pty faster than the
terminal can read. Therefore, make sure we read as much data as possible
but bail out early enough to not cause starvation.

Kernel TTY buffers are 4k, so reduce the overall buffer size, but read
more than once if possible (up to 8 times sounds reasonable).
console: add user console daemon

This adds a first draft of systemd-consoled. This is still missing a lot
of features and does some rather primitive rendering. However, it shows
the direction this code is going and serves as basis for further testing.

The systemd-consoled binary should be run as `systemd --user' unit. It
automatically picks up any session marked as Desktop=SYSTEMD-CONSOLE.
Therefore, you can use any login-manager you want (ranging from /bin/login
to gdm) to create sessions for systemd-consoled. However, the sessions
managers must be prepared to set the Desktop= variable properly.

The user-session is called `systemd-console', only the daemon providing
the terminal environment is called `systemd-consoled' (mind the 'd').

So far, only a single terminal session is provided on each opened
user-session. However, we support multiple user-sessions (even across
multiple seats) just fine. In the future, the workspace logic will get
extended so you can have multiple terminal sessions in a single
user-session for easier access.

Note that this is still experimental! Instructions on how to run it will
follow shortly.