Add support for setting process EUID/EGID

Add "-uid" and "-gid" CLI options to set real and/or effective user or group ID.
Remove useless XSEGBD_START/END options.
Introduce new conf section ARCHIPELAGO and add USER/GROUP options.
Switch peer processes to run as user USER and group GROUP.
Extend Peer class to support UID/GID options
Adjust tests according to the new user/group additions.
Add new configuration options, USER & GROUP.