Adopt C99 bool in the printing API.

Adopt C99 bool in the printing API.

Not many changes here: the 'dotted' flag passed to print_line_dotted
is bool, and so is the printing_in_colour flag passed to
print_get_colour. Also ps_init() takes a bool.

line_dotted is also a method in the drawing API structure, but it's
not actually filled in for any non-print-oriented implementation of
that API. So only front ends that do platform-specific _printing_
should need to make a corresponding change. In-tree, for example,
windows.c needed a fix because it prints via Windows GDI, but gtk.c
didn't have to do anything, because its CLI-based printing facility
just delegates to ps.c.