new module with runtime security checks, it will also check and do

new module with runtime security checks, it will also check and do

* libxslt/security.[ch] libxslt/Makefile.am: new module with
  runtime security checks, it will also check and do directory
  creation when allowed
* libxslt/documents.c libxslt/imports.c libxslt/transform.c
  libxslt/xslt.c libxslt/xsltInternals.h: plug-in the new
  security infrastructure probes at file reading or file creation
* xsltproc/xsltproc.c: plugged the security module there too,
  added the new options --nowrite and --nomkdir
* doc/*: updated the man page and regenerated.
Daniel