set-frame-size should respect line-spacing!?

[Ingo Lohmar]

In Bug#37563, I reported that `fit-frame-to-buffer' ignores a default
value of `line-spacing' > 0, whence the buffer might not be tall enough
--- that's what happened with the `posframe' pkg that is, eg, used by
`company-posframe' to display completions. Martin fixed this and related
issues in window.el quickly and thoroughly, for which I am grateful.

Since I tested with a minimal example, however, I did not realize before
that there is another code path that directly uses the C function
`set-frame-size', with the same effect: If line-spacing is > 0 in the
frame, it is not accounted for, and the frame lacks a few pixels in
height (number of lines * "line-spacing effect").

This appears to be a bug: If asked for a frame N lines tall, N lines (of
default-font's character height plus the effect of line-spacing) should
fit, right?

Maybe this affords some opportunity to concentrate this code on either
the lisp or the C side..?