Changed some types in mod_dom.c ; mostly 'xmlChar *' to 'const char *'.

Changed some types in mod_dom.c ; mostly 'xmlChar *' to 'const char *'.
The use of const is more appropriate than non-const becuase these
string references point to xmlNode content - and we are not allowed
to change that. Added buffer safe PI attribute reading for mod_dom.c by
implementing function attr_content_pi. Function index_value_of still has
potential buffer flows. The record extraction system now has a new member,
action, which may be modified by a record filter to signal
delete/replace/insert. This is only honoured if update is used (in which
case the outer system already has said "we don't care whether it's insert
or replace anyway). Added mod_dom test for the use for @type=delete .