oracle compatible func.

 Hi,

 the patch include:

    - rename ichar() to chr() (discussed with Tom)

    - add docs for oracle compatible routines:

        btrim()
        ascii()
        chr()
        repeat()

    - fix bug with timezone in to_char()

    - all to_char() variants return NULL instead textin("")
      if it's needful.

 The contrib/odbc is without changes and contains same routines as main
tree ... because I not sure how plans are Thomas with this :-)

                    Karel