small typo pointed out by Mike Hommey slightly improved the --c14n

* xmlIO.c: small typo pointed out by Mike Hommey
* doc/xmllint.xml, xmllint.html, xmllint.1: slightly improved
  the --c14n description, c.f. #144675 .
* nanohttp.c nanoftp.c: applied a first simple patch from
  Mike Hommey for $no_proxy, c.f. #133470
* parserInternals.c include/libxml/parserInternals.h
  include/libxml/xmlerror.h: cleanup to avoid 'error' identifier
  in includes #
* parser.c SAX2.c debugXML.c include/libxml/parser.h:
  first version of the inplementation of parsing within
  the context of a node in the tree #142359, new function
  xmlParseInNodeContext(), added support at the xmllint --shell
  level as the "set" function
* test/scripts/set* result/scripts/* Makefile.am: extended
  the script based regression tests to instrument the new function.
Daniel

test/scripts/set1.script

@@ -0,0 +1,2 @@
+set <b/>
+save -