New formula: lldpd, a 802.1AB implementation

lldpd is a 802.1AB implementation (LLDP). It discovers L2 neighbors
along with some information like hostname, port name, VLAN, etc.

Formula comes with support for launchd, creation of privilege
separation user and group and optional SNMP support.

The formula includes the use of dscl to create `_lldpd` user which is needed for privilege separation. No other current formula seems to have this need. A free UID is searched and the user still have to copy/paste the appropriate commands. Nothing needs to be done if the user already exists. The username follows standard convention on BSD and Mac OS X (starting by underscore, low UID).

No HEAD support since the bundled (but not used but still configured) libevent does not seem to support automake 1.13. I will add it once this is corrected.