lldpcli: Add support for JSON-C

OpenWRT is shipped with the JSON-C library, which is used for
several other packages. Having a separate JSON library (Jansson)
doing the exact same thing is wasting flash space.
cdp: Add power requirement to CDPv2 frames

LLDP already supports sending power requirements, for
IEEE 802.3at. Older Cisco PoE switches do not support
LLDP and need a custom CDPv2 frame. This patch enables
support if the LLDP MED PoE frame is also sent.
lldpcli: Add json description to lldpcli usage
cdp: Make it possible to enable CDPv2 without enabling CDPv1
cdp: complete manual page with CDP-related options
configure: if --with-json, default to jansson implementation
lldpcli: fix jansson implementation of the JSON output
NEWS: add a word about json-c library support