Fix auto-filling in Texinfo mode

Fix auto-filling in Texinfo mode

This fixes auto-filling in Texinfo buffers.  It was broken by the
fix to bug#49558, which made M-q fill over-long @noindent lines by
refraining from customizing 'paragraph-separate' in Texinfo mode.
The underlying problem here is that 'auto-fill-mode' doesn't call
mode-specific filling functions, but does its job by itself, and
depends on 'forward-paragraph' to find the beginning of the
paragraph as appropriate for calculation of 'fill-prefix', and a
different value of 'paragraph-separate' broke that.  As a side
effect, the change below also changes paragraph-movement commands
in Texinfo back to how they behaved prior to that bugfix, but I
don't see why the paragraph-movement behavior introduced by that
fix made more sense.  Try to move through a series of
@-directives, like a paragraph preceded by several @cindex
entries, and you will see the inconsistencies.  In any case, the
adverse effects of that fix on auto-filling is unacceptable.

* lisp/textmodes/texinfo.el (fill-paragraph-separate): New
variable.
(texinfo-mode): Set 'fill-paragraph-separate' to the default value
of 'paragraph-separate'.  Customize 'paragraph-separate' to the
Texinfo-specific value, as it was before commit dde591571abf.
(texinfo--fill-paragraph): Bind 'paragraph-separate' to the value
of 'fill-paragraph-separate', to keep 'M-q' happy.