Correspond to the make_mutable() ones.

One in particular is not a straight getter: hb_font_unset_funcs() is
special because of the specific needs of the lifecycle management of
the user_data object.

Helps with avoiding many "extern C" declarations in source files.

Also fixes issue with unicode callbacks never being set really.

Fixes lack of head_table initialization in create_for_tables() also.

hb_font_set_scale() now sets the value to be used to represent a unit
pixel.  For example, if rendering a 10px font with a 26.6 representation,
you would set scale to (10 << 6).  For 10px in 16.16 you would set it to
(10 << 16).  This space should be the same space that the get_glyph_metrics
and get_kerning callbacks work in.

In anticipation for buffer revamp coming.