bug#36860: SPC jumps to next page even before we are finished reading this page

[積丹尼 Dan Jacobson]

X-Debbugs-Cc: address@hidden
Package: w3m-el-snapshot
Version: 1.4.632+0.20190719-1

This no longer waits until we are at the bottom of a page to switch to
the next page:

     SPC runs the command w3m-scroll-up-or-next-url (found in
     w3m-mode-map), which is an interactive compiled Lisp function in
     ‘w3m.el’.

     It is bound to SPC.

     (w3m-scroll-up-or-next-url ARG)

     Scroll the current window up ARG lines, or go to the next page.

Now we are forced to use CTRL-V !

Test on long pages like:
file:///usr/share/doc/aptitude/html/en/ch02s04s05.html
$ dlocate /usr/share/doc/aptitude/html/en/ch02s04s05.html
aptitude-doc-en: /usr/share/doc/aptitude/html/en/ch02s04s05.html

In fact its docstring doesn't make clear that it only should go to the
next page if there is a next page (from <link rel> header), *and* we are
already at the bottom of this page.