Rewrite httpGetDateTime() to not use mktime(), since that function uses

Rewrite httpGetDateTime() to not use mktime(), since that function uses
the current timezone setting instead of UTC.

cups/http.c:
    - Rename days[] and months[] to http_days[] and http_months[] to
      avoid namespace issues.
    - httpGetDateString(): Use new http_days[] and http_months[].
    - httpGetDateTime(): Convert string to UTC using some
      relatively simple math instead of mktime().

cups/testhttp.c:
    - main(): Add tests for httpGetDateString() and
      httpGetDateTime() to verify that the new implementation
      works.

git-svn-id: svn+ssh://src.apple.com/svn/cups/cups.org/trunk@4636 7a7537e8-13f0-0310-91df-b6672ffda945