nnmail-cache-close should not use nofollow

nnmail-cache-close should not use nofollow

nnmail-cache-close did not work when ~/.nnmail-cache was a symlink
to some other directory.  Reported by Michael Albinus (Bug#39793).
* lisp/gnus/nnbabyl.el (nnbabyl-create-mbox):
* lisp/gnus/nndiary.el (nndiary-generate-nov-file):
* lisp/gnus/nnfolder.el (nnfolder-possibly-change-group):
* lisp/gnus/nnmbox.el (nnmbox-create-mbox):
* lisp/gnus/nnml.el (nnml-generate-nov-file):
Pass ‘excl’ to nnmail-write-region when creating a file, for safety.
* lisp/gnus/nnmail.el (nnmail-write-region):
Add optional MUSTBENEW arg, to stay consistent with write-region.
Past ‘nofollow’ to set-file-modes only when MUSTBENEW is ‘excl’.