New lispref section “Security Considerations”

New lispref section “Security Considerations”

This attempts to document some of the issues recently discussed
on emacs-devel, and to indicate other such issues.  The section
could be a lot longer.
* doc/lispref/os.texi (Security Considerations):
New node.
* doc/lispref/elisp.texi (Top):
* doc/lispref/processes.texi (Shell Arguments):
* lisp/subr.el (shell-quote-argument):
* src/callproc.c (syms_of_callproc):
Reference it.