Multiple instances support added

Multiple instances support added

Now multiple MongoDB instances may be started by our mongod init script.
Each instance is configured via /etc/mongod/${name}.conf.

Old configuration from /etc/mongod.conf is moved to
/etc/mongod/default.conf on upgrade. Database, pid file and log file
locations are not changed for the default instance.

Non-default instances have their pid file changed to
/var/run/mongod-${name}.pid. Database and log file locations are defined
in the /etc/mongod/*.conf files and it is the administrator
responsibility to set them right.