bug#4616: 23.1.50; empty script files

[Markus Rost <rost@math.uni-bielefeld.de>]

What is the best way to implement --script?  Let me discuss some
possibilities:

1. The current implementation.  It consists of the lines

      if (c == '!')
	{
	  /* #! appears at the beginning of an executable file.
	     Skip the first line.  */
	  while (c != '\n' && c >= 0)
	    c = READCHAR;
	  goto retry;
	}

   in function read1 in lread.c.  This is appealing by its brevity.
   However it has the disadvantage that the basic read function has a
   *general* feature in order to get --script and the "#!"-syntax
   working.  Further it leads to the bug originally posted:  Execution
   of an empty script file like

   ===File ~/x.el==============================================
   #!/home/rost/build/cvs/emacs/src/emacs --script
   ============================================================

   yields an "End of file during parsing" error and returns with positive
   exit status.

2. One may think of handling --script by first removing a first
   #!-line before passing the file to load.  This seems to be very
   hard to implement because the load routine is complicated
   (switching back and forth between C and Elisp functions).  Besides
   this practical problem, there is the following disadvantage:  In
   some circumstances one may want to load a script file directly, as
   in

   emacs -batch ... -l ~/x.el ...

   Moreover, when editing the file one may want to test it with
   eval-buffer.

3. This leaves the possibility to handle #!-lines when running
   eval-region.  It should skip a #!-line at the beginning of the
   region.  It could skip any #!-line in the region, which is what
   read1 does right now, but I see no necessity for that.  This would
   be implemented by my two previous patches

   <URL:http://lists.gnu.org/archive/html/bug-gnu-emacs/2009-10/msg00082.html>
   <URL:http://lists.gnu.org/archive/html/bug-gnu-emacs/2009-10/msg00124.html>

   Of course making a change in readevalloop/read1 needs a careful
   check, maybe by several people.  However, the patch is a very
   "local" one around the variable unread_char and there are only a
   few places where unread_char appears, so checking the patch is not
   too much work.  Moreover, if something would be wrong with the
   change, very likely emacs would not even compile as many files are
   loaded during the build process.