peer: Add support for setting process EUID/EGID

Add "-uid" and "-gid" CLI options to set real and/or
effective user or group ID.
conf: Remove useless XSEGBD_START/END options
conf: Introduce new options USER/GROUP

Introduce new conf section ARCHIPELAGO and add USER/GROUP options.
Switch peer processes to run as user USER and group GROUP.
archipelago: Extend Peer class to support UID/GID options
tests: Fix tests

Adjust tests according to the new user/group additions.
docs: Update docs based on the new block device

Add new configuration options, USER & GROUP.
Switch license to GPLv3

According to the decision of the GRNET Board of Directors,
switch license to GPLv3.

This commit will be propagated to the release
and master branches based on git flow, and the next
release will be licensed as GPLv3.
mapper: Fix v2 mapdata layout

Introduce new v2 mapdata layout. The new layout introduces restrictions on the
size of the object on the map, but it is simpler and does not split objects
between mapdata blocks. It also behaves correctly with small blocksizes.