xml.c, config,c: Add stylesheets and variable list string parsing

Added functions to open, close, and apply XML Stylesheets
to XML documents.  Although the presence of libxslt was already
being checked by configure, it was only happening if xmldoc was
enabled.  Now it's checked regardless.

Added ability to parse a string consisting of comma separated
name/value pairs into an ast_variable list.  The reverse of
ast_variable_list_join().

Change-Id: I1e1d149be22165a1fb8e88e2903a36bba1a6cf2e