OverrideOutgoingMailFrom: Use $CorrespondAddress if the queue's isn't set

Otherwise the meaning of the Default key changes depending on the
queue's correspond address being set or not.  Using the global
correspond address for queues with no specific correspond address, as
well as moving the Default check into the else branch, means the Default
key is only used when there's no ticket involved.  This is the
documented behaviour in RT_Config.pm.
SetOutgoingMailFrom: Accept an email address as a value and use it globally

Covers the simple case of sending all bounces to a specific address,
without the only previous solution of resorting to defining all queues
in OverrideOutgoingMailFrom.  Any definitions in the Override option
still take precedence.
Merge branch '4.0/outgoing-mail-from' into 4.0-trunk

Conflicts:
	etc/RT_Config.pm.in