Fix bug #17801 with extraneous newlines after inserting markup in Texinfo mode.

Fix bug #17801 with extraneous newlines after inserting markup in Texinfo mode.

 lisp/textmodes/texinfo.el (texinfo-mode): Set skeleton-end-newline
 locally to nil.
 (texinfo-insert-block, texinfo-insert-@end)
 (texinfo-insert-@example, texinfo-insert-@quotation): Adjust to
 local setting of skeleton-end-newline by adding an explicit \n to
 the skeletons where appropriate.