[VM] headers only mode for local folders

[John Hein]

I like [the fairly recent] headers only mode for imap folders.

What about for local folders?  That would help for some of my larger
local folders, especially for load/save operation times (especially
over nfs) and searching based on header contents (rather than body
searches).

This might help mitigate some (not all) of the reason I'd like to see
maildir support in vm.  Although I'm not sure which would be easier
to implement.