Add support for random Face headers

Add support for random Face headers

* gnus-fun.el (gnus-x-face-omit-files): Regexp to omit matched results
from random face commands.
(gnus-face-directory): Like `gnus-x-face-directory` for png files and
Face.
(gnus-face-omit-files): Like `gnus-x-face-omit-files` for Face.
(gnus--random-face-with-type): Generic function returning a face-type
as a string.
(gnus--insert-random-face-with-type): Generic function inserting a face
in a message buffer header.
(gnus-random-x-face): Rewritten to use `gnus--random-face-with-type`.
(gnus-insert-random-x-face-header): Rewritten to use
`gnus--insert-random-face-with-type`.
(gnus-random-face): Return random (png) Face as string.
(nus-insert-random-face-header): Insert random (png) Face in a message
buffer.