(sh-newline-and-indent): Don't interpret prefix arg

(sh-newline-and-indent): Don't interpret prefix arg
since `newline-and-indent' doesn't either.  Don't turn a trailing
quoted tab into a space.
(sh-mode): Use new `skeleton-newline-indent-rigidly'.
(sh-set-shell): Make maximum font-locking also hightlight keywords
after ``' and `!'.  (The latter is for ksh '93 but should't hurt other
shells.)