terminal: move unifont-internal.h to unifont.h

All the definitions are for outside users, so drop the -internal suffix.
Internal definitions are in unifont-def.h and unifont.c, no need to share
those.
terminal: add unifont_get_width/height()

Allow unifont users to retrieve the width and height of unifont glyphs. In
version 1 this is hard-coded as 8/16, but may be changed in the future.
terminal: move unifont-map to datadir

Lets avoid putting stuff into /usr/shared/unifont/, but keep it in
/usr/share/systemd/. Upstream lacks interest in this, so don't bother for
now.
terminal: add term.h header for library users

Like all the other parts of libsystemd-terminal, split API of
term-internal.h into term.h so we can use it from systemd-consoled.
terminal: add helpers to retrieve page dimensions

Allow term users to retrieve the page dimensions of a terminal screen.
This is needed to properly calculate the grid dimensions when rendering.
barrier: fix up constructor error handling

We cannot rely on "errno" to be non-zero on failure, if we perform
multiple glibc calls. That is, if the first eventfd() call fails, but the
second succeeds, we cleanup the barrier but return 0.

Fix this by always testing the return value immediately. This should also
fix all the coverity warnings.