The default initiator IQN is "iqn.2000-09.org.etherboot:UNKNOWN".
This is problematic for two reasons:

  a) the etherboot.org domain (and hence the associated IQN namespace)
     is not under the control of the iPXE project, and

  b) some targets (correctly) refuse to allow concurrent connections
     from different initiators using the same initiator IQN.

Solve both problems by changing the default initiator IQN to be

  iqn.2010-04.org.ipxe:<hostname> if a hostname is set, or

  iqn.2010-04.org.ipxe:<uuid> if no hostname is set.

Explicit initiator IQNs set via DHCP option 203 are not affected by
this change.

Unfortunately, this change is likely to break some existing
configurations, where ACL rules have been put in place referring to
the old default initiator IQN.  Users may need to update ACLs, or
force the use of the old IQN using an iPXE script line such as

  set initiator-iqn iqn.2000-09.org.etherboot:UNKNOWN

or a dhcpd.conf option such as

   option iscsi-initiator-iqn "iqn.2000-09.org.etherboot:UNKNOWN"

Signed-off-by: Michael Brown <mcb30@ipxe.org>