change to default archive/delete key bindings

[Jameson Graef Rollins]

While working on the delete message handling patches, I was reminded
how much I really dislike the default show-mode key bindings.  Why
can't I just archive/delete the current message, without archiving the
entire thread?  It doesn't make any sense.

Here we add two new functions to archive and delete just the single
message, and then move to the next open message.  We also add an
option to the -next-open-message function so that it will pop back out
to the parent search buffer when reaching the end of the thread.  This
should make message processing flow much smoother.

Patches 1,2 and 4 can be applied even if the consensus is to not
change the default key bindings, to make it easier for users to
achieve the desired functionality without having to write their own
functions.

jamie.