Follow-up to 40c264db617d0 after discussions on IRC.
The new style is
name[0-99]=contents
and
name[0-99]@filename
A) This does not cause the same problems with old curl versions trying
the new syntax as this way will cause old curls just fail with syntax
error and not risk using the wrong file.
B) Adds the same byte range support for "normal" assigns, which the
previous syntax did not. Thus lets a user get a partial content of a
variable etc.
Added test 790 and 791 to verify non-file assigns with ranges.
Closes#15862
