Added write buffering to the HTTP code to improve performance

Added write buffering to the HTTP code to improve performance
(STR #547)

The scheduler now uses the attributes-natural-language attribute
to localize banner pages (STR #386)

cups/http.c:
    - httpFlushWrite(): Added.
    - httpWrite(): Use write buffer for small writes.
    - httpPrintf(): Use write buffer for small writes.
    - httpUpdate(): Flush write buffer as needed.

cups/http.h:
    - Change buffer size to 1024 bytes.
    - Add wbuffer and wused fields to http_t structure.
    - Add httpFlushWrite() function.

scheduler/client.c:
    - CloseClient(): Flush write buffer as needed.
    - WriteClient(): Flush write buffer as needed.

scheduler/ipp.c:
    - copy_banner(): Use attributes-natural-language instead of
      con->language to localize banners.

git-svn-id: svn+ssh://src.apple.com/svn/cups/cups.org/trunk@4675 7a7537e8-13f0-0310-91df-b6672ffda945