ignore specials in terms without `|`

A similar example came up in `GLMpack` tests:

```r
library(lme4)
cs <- function(x) scale(x, center=TRUE, scale = TRUE)
lmer(Reaction ~ cs(Days) + (1|Subject), sleepstudy)
```

> Error in x[[3]] : object of type 'symbol' is not subsettable

In the short term `GLMpack` maintainers can work around this (by naming the utility function `cs0` or something else), but `reformulas` should be fixed to ignore this ...

Start by reproducing the GLMpack example with the reformulas formula-processing code, focusing on the term `cs(Days)` when no `|` appears in the formula. The fix is done when this expression no longer raises the symbol-subsetting error and random-effect formulas continue to work.

Written by the indexing model from the issue text.