Added "user" and "group" options to general section of configuration file.
NFD will attempt to set the effective group and user id to these values
after initializing all management modules.
Added privilege helper to drop and temporarily elevate privileges on demand.
Updated README.md with instructions to configure NFD to drop privileges.
Added handler for general confguration file section.
refs: #1370
Change-Id: Id27140ad2dc2ca14751058691511132a35649d58
Alexander Afanasyev
Eric Newberry
Steve DiBenedetto
Alexander Afanasyev