[R6RS] script syntax

[dyb at cs.indiana.edu]

> I've committed a change to scripts.tex to allow for alternate script 
> header lines.
>
> Kent mentioned the idea of installation programs performing 
> platform-specific rewrites of script headers, but specifying that seems 
> like a can of worms to me.  I'm open to suggestions.

My intent was to allow the first line of a script header to be any line
starting with "#!<space>" or "#!/" and require that the script loader
ignore such a line when present.  Otherwise, "#!<space>" and "#!/" would
be invalid syntax.

The current lex.tex treatment of #!/usr/bin/env as a comment---which is
incorrect anyway because it doesn't extend to the end-of-line as
written---would go away, and the "#!<space>" and "#!/" headers would be
mentioned only in scripts.tex.

Kent