Use Mukti as a fallback font for Bangali

It fits sylistically with DejaVu and has both a normal and bold version
Support more recent unifont versions

Unifont's name changed from "unifont" to "Unifont" at some point,
probably with 6.0.

This commit causes the more recent caps to be preferred, but will
support either version.

Closes gravitystorm/openstreetmap-carto#429