Bug 995202: Rename LLCP packets to PDUs

The LLCP specification calls individual LLCP messages 'PDUs', so
we should too.

Signed-off-by: Thomas Zimmermann <tdz@users.sourceforge.net>
Bug 995202: Added LLCP SAP constants to NFC emulation

The SAP constants contain magic numbers for the Service Access
Points of LLCP, as defined by the NFC forum.

Signed-off-by: Thomas Zimmermann <tdz@users.sourceforge.net>
Bug 995202: Cleanup NFC buffer reading/writing

This patch adds a nice API around NFC-internal buffers.

Signed-off-by: Thomas Zimmermann <tdz@users.sourceforge.net>