Setting the identity

[Peter Bloomfield <PeterBloomfield MindSpring com>]

Setting identities has a minor inconsistency; perhaps this has been 
discussed--I don't recall.

The `mailbox->identity_name' field is set for all mailboxes loaded from 
configuration, using

     mailbox->identity_name = 
gnome_config_get_string("Identity=Default");

If the config has no "Identity" string, the name defaults to "Default". 
Other mailboxes, found by scanning a folder tree, have a NULL value for 
`mailbox->identity_name'.

Now as it happens, I have an identity named "Default" with no fields 
set. So if I answer a message in a mailbox loaded from config, the 
message identity is set to this useless value, whereas if I answer a 
message in any other mailbox, it is guessed from the sender and 
defaults to the identity that *I* set as the default.

IMHO it would be more consistent to let the `mailbox->identity_name' 
default to NULL when loading mailboxes from config. Does anyone really 
need the current behavior?