Add support for more bases in $GENERATE

Add support for more bases in $GENERATE

When reading a zone from a zonefile, the `$GENERATE` resolution now not
only supports decimal but also octal, hexadecimal and nibbles.

When using nibbles with an even width, the generated index may end with
a dot, and alone is interpreted as a absolute name. This behaviour is
consistent with bind, but may cause these records to be dropped by the
subdomain-check in `zonefile.py:398` (see also the `h.*` labels in the
testcase that are missing from the result).