Make seed = I(FALSE) escalate to an error if RNG was used

Currently, the default behavior of declaring use of the random number generator (RNG) is `seed = FALSE`, which triggers a warning if it detects that the future did indeed use the RNG despite not declaring so;

```r
library(future)
f <- future(sample.int(3)) ## defaults to seed = FALSE
v <- value(f)
v
#> [1] 3 2 1
#> Warning message:
#> UNRELIABLE VALUE: Future () unexpectedly generated random numbers without specifying argument 'seed'. There is a risk that those random numbers are not statistically sound and the overall results might be invalid. To fix this, specify 'seed=TRUE'. This ensures that proper, parallel-safe random numbers are produced. To disable this check, use 'seed=NULL', or set option 'future.rng.onMisuse' to "ignore". [future (54b70f088e2c65b835aea5f0cd100eff-1); on 54b70f088e2c65b835aea5f0cd100eff@hb-x1-2023<867157>]
```

We can escalate these type of warnings to errors globally by setting R option `future.rng.onMisuse` as in:

```r
library(future)
options(future.rng.onMisuse = "error")
f <- future(sample.int(3))
v <- value(f)
#> Error: UNRELIABLE VALUE: Future () unexpectedly generated random numbers without specifying argument 'seed'. There is a risk that those random numbers are not statistically sound and the overall results might be invalid. To fix this, specify 'seed=TRUE'. This ensures that proper, parallel-safe random numbers are produced. To disable this check, use 'seed=NULL', or set option 'future.rng.onMisuse' to "ignore". [future (54b70f088e2c65b835aea5f0cd100eff-2); on 54b70f088e2c65b835aea5f0cd100eff@hb-x1-2023<867157>]
```

The problem with this approach is that options are global and affects all codes, even where we don't want a strict check. Also,none of the future R options should be set developers - only end-users for troubleshooting etc. One can of course set options temporarily to control this, but it adds clutter and is error prone.

A better way to give the developer control of the severity of RNG misuse is via the `seed` argument itself. Here are a few approaches:

* `seed = structure(FALSE, onMisuse = "error")`
* `seed = I(FALSE)` - "as-is"

We could also move towards making `seed = FALSE` be strict and give an error, and `seed = NA` to be the current soft warning. However, such a migration is a much bigger task given that we'll break some existing code.

No files or tests are named. Start by tracing the future() seed argument and the existing future.rng.onMisuse handling; compare the proposed per-call severity forms and define compatibility with seed=FALSE and global options. Done means the selected behavior is specified and verified by tests.

Written by the indexing model from the issue text.