Added docs for 'user' customization module. Renamed libuser.tex

(which had UserDict/UserList) to libuserdict.tex.

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+\section{Standard Module \sectcode{user}}
+\label{module-user}
+\stmodindex{user}
+\kwindex{.pythonrc.py}
+
+As a policy, Python doesn't run user-specified code on startup of
+Python programs.  (Only interactive sessions execute the script
+specified in the \code{PYTHONPATH} environment variable if it exists).
+
+However, some programs or sites may find it convenient to allow users
+to have a standard customization file, which gets run when a program
+requests it.  This module implements such a mechanism.  A program
+that wishes to use the mechanism must execute the statement
+
+\bcode\begin{verbatim}
+import user
+\end{verbatim}\ecode
+
+The user module looks for a file \file{.pythonrc.py} in the user's
+home directory and if it can be opened, exececutes it (using
+\code{execfile()}) in its own (i.e. the module \code{user}'s) global
+namespace.  Errors during this phase are not caught; that's up to the
+program that imports the user module, if it wishes.  The home
+directory is assumed to be named by the \code{HOME} environment
+variable; if this is not set, the current directory is used.
+
+The user's \file{.pythonrc.py} could conceivably test for
+\code{sys.version} if it wishes to do different things depending on
+the Python version.
+
+A warning to users: be very conservative in what you place in your
+\file{.pythonrc.py} file.  Since you don't know which programs will
+use it, changing the behavior of standard modules or functions is
+generally not a good idea.
+
+A suggestion for programmers who wish to use this mechanism: a simple
+way to let users specify options for your package is to have them
+define variables in their \var{.pythonrc.py} file that you test in
+your module.  For example, a module \code{spam} that has a verbosity
+level can look for a variable \code{user.spam_verbose}, as follows:
+
+\bcode\begin{verbatim}
+import user
+try:
+    verbose = user.spam_verbose  # user's verbosity preference
+except AttributeError:
+    verbose = 0                  # default verbosity
+\end{verbatim}\ecode
+
+Programs with extensive customization needs are better off reading a
+program-specific customization file.
+
+Programs with security or privacy concerns should \emph{not} import
+this module; a user can easily break into a a program by placing
+arbitrary code in the \file{.pythonrc.py} file.
+
+Modules for general use should \emph{not} import this module; it may
+interfere with the operation of the importing program.
+
+For a site-wide customization mechanism, see module \code{site}.