rebase + minor changes:
* remove unnecessary volatile qualifier from a variable only read by
PG_TRY and never accessed by PG_CATCH or PG_END_TRY
* remove old comment from xml.sql and unnecessary entry from
system_functions.sql (leftovers from last implementation)
* add the c14n enum XML_C14N_1_1 instead of its value in the
xmlC14NDocDumpMemory() call.
Jim