til_args: add --seed= explicit PRNG seeding support

This enables reproducible yet pseudo-randomized visuals, at least
for the fully procedural modules.

The modules that are more simulation-y like sparkler and swarm
will still have runtime variations since they are dependent on
how much the simulation can run and there's been a lot of
sloppiness surrounding delta-t correctness and such.

But still, in a general sense, you'll find more or less similar
results even when doing randomized things like
module=rtv,channels=compose using the same seed value.

For the moment it only accepts a hexadecimal value, the leading
0x is optional.

e.g. these are all valid:
--seed=0xdeadbeef
--seed=0xdEAdBeFf
--seed=0x	(produces 0)
--seed=0xff
--seed=deadbeef
--seed=ff
--seed=		(produces 0)
--seed=0	(produces 0)

when you exceed the natural word size of an unsigned int on your
host architecture, an overflow error will be returned.

there are remaining issues to be fixed surrounding PRNG
reproducibility, in that things like til_module_randomize_setup()
doesn't currently accept a seed value.  However it doesn't even
use rand_r() currently, but when it invokes desc->random() the
module's random() implementation should be able to use rand_r()
and needs to be fed the seed.  So that all still needs wiring up
to propagate the root seed down everywhere it may be relevant.

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