Use Droid Fallback as a fallback before Unifont

Feedback is that Droid Fallback looks better than Unifont. As Droid
Fallback doesn't have bold, hopefully we don't get this far, but
it beats Unifont.

The droid fonts can be found in the fonts-droid package on Ubuntu.

A readme section to list required font packages has also been added
Add Arundina Sans to font list

Arundina Sans is based on DejaVu and has bold and italic versions, so it's an easy choice. Obtained with the ``fonts-sipa-arundina`` package.

This change makes mapnik/mapnik#2161 more obvious, but we were probably encountering that bug before with Unifont, it's just hard to tell with how hard Unifont was to read.
Add Padauk to font list

Padauk has both a normal and a bold version, making it a good choice.

It's also the only packaged Burmese font I could find.
Add text explaining what happens with missing fonts and how only fonts for languages you care about are required
Add Khmer OS Metal Chrieng Regular to font list

It's been recommended by a native Khmer speaker, and is packaged
on Ubuntu.

Also split the font lists into multiple lines because it's getting long
Unify road definitions for bridges, tunnels, and normal roads

Code for every highway type is currently spread out over three places in
the code: once for bridges, once for tunnels, and once for normal roads.
This means that a lot of code is duplicated. The current pull request
brings the code for a single highway type together at one place in the
code.

Although the code is brought together, it does not yet collapse all
equivalent definitions. This will be done in the next pull request.

The advantage of this is easier maintenance, because a) the CartoCSS
code will be shorter; b) there will be less duplication in the code;
and c) code for one type of road is at only one location in the code.

Changes
-------
Although the aim of this change is to improve the structure of the code,
it has some small changes of rendering as side effects:
- The tag access=no is no longer rendered on railways, aeroways, and
  roads under construction.
- For bridges and tunnels, access restrictions are rendered at the same
  time as the roads to which they apply, just like it was already the
  case for normal roads.
- Direction arrows are now layered as own attachment, but still directly
  on top of the layer they apply to.
- Tunnels and bridges that are a link are now rendered under other roads
  of the same osm-layer, just like links that are not a tunnel or bridge.
- The following kind of tunnels are now correctly rendered in the tunnel
  layer instead of in the regular layer: the highway types tertiary
  (z10-12), residential (z10-12), unclassified (z10-12), service,
  living_street, road (fill only), road (z10-12), pedestrian, platform,
  steps, track, raceway, construction, proposed; all railway types; all
  aeroway types.
- The following kind of bridges are now correctly rendered in the bridge
  layer instead of in the regular layer: the highway types secondary
  (z12), tertiary (z10-13), unclassified (z10-13), residential (z10-13),
  road (z10-13), living_street, pedestrian (z13-15), bridleway (z13),
  footway (z13), cycleway (z13), path (z13), track (z13), raceway,
  construction, platform, steps; the railway types spur (z11-12), siding
  (z11-12), yard (z11-12), funicular, minitiature, tram, lightrail
  (z13), subway (z12-13), preserved, monorail, platform, turntable; the
  aeroway type runway (z11-13).
- Ways with both tunnel=yes and bridge=yes might get rendered different
  than before (but such ways should not exist anyway).
- highway=road is now also rendered on the layer for bridges and
tunnels.
- One-way restrictions on bridges are no longer displayed on highways
types that are not rendered.
- One-way restrictions on bridges are now displayed on all rendered
railway types.

Performance
-----------
In general performance seems to improve. I compared performance between
2254ec971ca418e34295c67417febf003c5182cc and
42e4203f2fd9a40e9732b59db0a97506e399ec96. The resulting output osm.xml
is now 8% smaller. I measure a performance loss of 4% at z15, but
performance gains of 12%, 16%, and 13% on z16, z17, and z18,
respectively. My measurements are not very accurate, so I would be happy
if @pnorman or someone else could independently verify this.

Pull request size
-----------------
I'm sorry for the huge pull request. I couldn't break it up without
breaking functionality or significantly increasing run-time. The next
pull requests will be smaller.
Merge branch 'roadsv5cnew' of https://github.com/math1985/openstreetmap-carto into math1985-roadsv5cnew
Add rendering rules for missing road names.
Add labels for path types.
Amend railways in where clause.

Some features disappeared after the refactoring, so this at least
makes the where clause match the case clause entries.

Fixes #364