ddb(4): Add some support for lexing IPv6 addresses

Allow commands to specify that (hex) numbers may start with A-F, by adding
the DRT_HEX flag for db_read_token_flags(). As before, numbers containing
invalid digits for the current radix are rejected.

Also, lex ':' and '::' tokens as tCOLON and tCOLONCOLON respectively.

There is a mild conflict here with lexed "identifiers" (tIDENT): ddb
identifiers may contain arbitrary colons, and the ddb lexer is greedy. So
the identifier lex will swallow any colons it finds inside identifiers, and
consumers are still unable to expect the token sequence 'tIDENT tCOLON'.
That limitation does not matter for IPv6 addresses, because the lexer always
attempts to lex numbers before identifiers.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D21509