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.