Moved author and date to boilerplate.tex.

Added copyright.tex to all files.
Added descr of check_interval to libsys.tex.
Corrected lotsa typos in tut.tex (thanks to George M. Sipe).
Corrected mktuple (should be newtupleobject) example in ext.tex.

Doc/libsys.tex

@@ -20,6 +20,15 @@ It is always available.
   modules.)
 \end{datadesc}
 
+\begin{datadesc}{check_interval}
+When this variable is set to an integer value, it determines how often
+the interpreter checks for periodic things such as thread switches and
+signal handlers.  The default is 10, meaning the check is performed
+every 10 Python virtual instructions.  Setting this to a large value
+may increase performance for programs using threads.  Setting it to a
+value <= 0 checks every virtual instruction, maximizing responsiveness
+as well as overhead.
+
 \begin{datadesc}{exc_type}
 \dataline{exc_value}
 \dataline{exc_traceback}