Decode Gnus group names when browsing server group lists

* lisp/gnus/gnus-srvr.el (gnus-browse-foreign-server): The coding
  system for the nntp-server-buffer is 'binary; decode group names as
  'utf-8-emacs.
1 job for scratch/gnus-decoded in 54 minutes and 31 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #1511
test-all

00:54:31