skeleton, skelrc, skelrc.script: Variable shebang lines.

skeleton, skelrc, skelrc.script: Variable shebang lines.

Tweak the general skeleton to allow a completely variable first line
(defaulting to `new-comment' as before).  Introduce a new `skelrc.script'
which fills in a shebang line instead, and sets the major mode from it
using `interpreter-mode-alist' in the proper way.