core: add support for temporary privilege drop and elevation

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
nfd-status: Define format and implement --xml option

The ether URI format "ether://01:00:5e:00:17:aa" violates
the URI specification, so it has been changed to
ether://[01:00:5e:00:17:aa] in this commit.

refs: #1438

Change-Id: I1218374b75f919a5e1ab158bbab2e217e64aced3
core: Fixing regression in PrivilegeHelper that prevented running as non-root

PrivilegeHelper was properly initialized only when general section was
present.  When section was omitted, s_privilegedUid/s_privilegedGid/s_normalUid/s_normalGid
variables was not properly initialized (set to 0), resulting in
exception during the subsequent PrivilegeHelper::drop call.

Change-Id: Iffd7f05e7e6c86d227da2c9b7920ca6e3b607211