[xml] modifying tree and attributes

[Rob Richards <rrichards ctindustries net>]

I was looking at bug 319108 (which I agree with assement and additional 
function needed) and was adding in some additional fixes from code I use.
i.e. - dont unlink or free if XML_ATTRIBUTE_DECL returned from xmlHasXXXProp
   - dont add attr if #FIXED in DTD

These changes affect xmlAddNextSibling, xmlAddPrevSibling and 
xmlAddChild. imo these are higher level functions so all checks should 
be done, but...

Where is the line drawn on what is expected the caller to handle and 
what the lib handles?
Looking at a few other functions:
xmlNewXXXProp - imo up to caller to handle checks as code does no checks 
for anything so no issues here.
xmlSetXXXProp - grey area here. function checks for exsiting attribute 
and IDs but not XML_ATTRIBUTE_DECL (really only #FIXED would cause any 
problems). But in the case ofsetting a prop with ns, it is up to the 
caller to insure ns is in scope. so basically some things handled here 
while others arent.

Rob