core: fix pointer caps, slowpath kbd and a gcc fix

* According to MS-RDPBCGR 2.2.7.1.5 the pointerCacheSize is optional
  and its absence or a zero value indicates missing client support for
  the New Pointer Update.

* Added and fixed some comments regarding the meaning of the KBDFLAGS_DOWN
  keyboard flag and how it is currently used in the code.
  "Fixed" the slow path keyboard input to generate the same keyboard flags
  as the corresponding fast path code.

* Some arbitrary value was used for the ConnectPDULength in the GCC
  Conference Create Response. According to MS-RDPBCGR 4.1.4 this value must
  be ignored by the client so we encode a zero value instead.
Merge pull request #2160 from nfedera/fix-2014-10-22-01

core: fix pointer caps, slowpath kbd and a gcc fix