Improve use of boolean defaults (scala#23564)

A couple of cases where recursing with default booleans was especially
confusing.

Spellcheck would like to change booleans to beanpoles.

I know the project hates so-called style PRs.

But if I see something morally wrong, I cannot but act upon it.

I must confirm that inlined helper such as `jsig1` allows `tailrec`.
Maybe it's enough to annotate jsig? Edit: I see goto 0 only. With
tailrec, it complains about the RHS of the helper `inline def jsig1`,
which is spurious.
[Cherry-picked 3770ee8][modified]

Author: tgodzik