ddb(4): Enhance lexer functionality for specialized commands

Add a db_read_token_flags() variant of db_read_token() with configurable
parameters.

Allow specifying an explicit radix for tNUMBER lexing. It overrides the
default inference and db_radix setting.

Also provide the option of yielding any lexed whitespace (tWSPACE) (instead
of ignoring it). This is useful for whitespace-sensitive CS_OWN commands.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D21459