Include rand-isaac.c rather than rand-isaac.h.

Include rand-isaac.c rather than rand-isaac.h.
Don't include md5.h; it wasn't needed.
(struct keyfield): Rename random_hash to random, for consistency
with the other member names.  All uses changed.
(usage): Tweak wording to mention STRING for --seed option.
(short_options): Rorder for consistency with other programs.
(rand_state): Now a struct, not a pointer to one.  All uses changed.
(HASH_WORDS, HASH_SIZE): Remove.
(get_hash): Remove comments around resbuf size, since we can assume C89.
Use a "more-kosher" (but slower) approach of invoking isaac_refill.
(keycompare): Adjust to the new get_hash.
Add a FIXME.
(badfieldspec): Omit recently-introduced comment; it isn't needed.
(main): Don't set need_random simply because gkey has it set; that
doesn't necessarily mean we'll need random numbers.
Redo seeding to match new get_hash approach.