Refactor: find_site_by_name to no longer rely on global booth_conf var

Also, apply the same recursively to both the underlying and superjacent
call graph expansion (respective static functions for the former,
progression up to main.c, i.e. do_attr_command, for the latter).

Also, exercise some const-correctness at places being touched,
and mark unused function as such.

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