Move verifyzone() and its dependencies into lib/dns/zoneverify.c

Move verifyzone() and its dependencies into lib/dns/zoneverify.c

This commit only moves code around, with the following exceptions:

  - the check_dns_dbiterator_current() macro and functions
    is_delegation() and has_dname() were removed from
    bin/dnssec/dnssectool.{c,h} and duplicated in two locations:
    bin/dnssec/dnssec-signzone.c and lib/dns/zoneverify.c; these
    functions are used both by the code in bin/dnssec/dnssec-signzone.c
    and verifyzone(), but are not a good fit for being exported by a
    code module responsible for zone verification,

  - fatal() and check_result() were duplicated in lib/dns/zoneverify.c
    as static functions which do not use the "program" variable any more
    (as it is only set by the tools in bin/dnssec/); this is a temporary
    step which only aims to prevent compilation from breaking - these
    duplicate functions will be removed once lib/dns/zoneverify.c is
    refactored not to use them,

  - the list of header files included by lib/dns/zoneverify.c was
    expanded to encompass all header files that are actually used by the
    code in that file,

  - a description of the purpose of the commented out "fields" inside
    struct nsec3_chain_fixed was added.