terminal: only build if --enable-terminal was specified

Whoopsy, I totally forgot adding the "if ENABLE_TERMINAL" markers. Do that
now that we know it builds fine everywhere.
terminal: add unifont font-handling

The unifont layer of libsystemd-terminal provides a fallback font for
situations where no system-fonts are available, or if you don't want to
deal with traditional font-formats for some reasons.

The unifont API mmaps a pre-compiled bitmap font that was generated out of
GNU-Unifont font-data. This guarantees, that all users of the font will
share the pages in memory. Furthermore, the layout of the binary file
allows accessing glyph data in O(1) without pre-rendering glyphs etc. That
is, the OS can skip loading pages for glyphs that we never access.

Note that this is currently a test-run and we want to include the binary
file in the GNU-Unifont package. However, until it was considered stable
and accepted by the maintainers, we will ship it as part of systemd. So
far it's only enabled with the experimental --enable-terminal, anyway.
terminal: suppress warning in subterm

Empty format-strings are just fine if format-functions do more than
printing. This is the case here, so suppress the "empty format-string"
warning by using "%s" with an empty argument.