[emacs-w3m:13457] New branch 201906

Hi,

I made a new branch named `modernize201906' mainly to drop old
Emacsen and XEmacs support.  Now it supports Emacs 25,26 and 27.
It doesn't necessarily mean to make it fast but clean the code.
Still uses lexical-let, using the lexical-binding would be the
next step.  Testers are welcome.  Note that you may want to do
`git fetch' to check out the branch.

The new w3m-download implementation ([emacs-w3m:13326]) is added
as a makeshift until the improved w3m ([emacs-w3m:13327]) comes.

Tested and tweaked the code so that `make slow' with Emacs 25.1,
26.1, 26.2.90, and 27.0.50 may issue no warning.  Mew stuff are
tested by only byte-compiling along with the latest its git repo.

Regards,