Refactor: FOREACH_{NODE,TICKET} to no longer rely on global booth_conf

Also, apply the same recursively to superjacent call graph expansion
(progression up to main.c), and apply that whenever it is not eligible
where it previously was not (because FOREACH_{NODE,TICKET} not accepting
struct booth_config * injection.

Also, exercise some const-correctness at places being touched,
and hide some functions as merely static ones.

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>