Add $resume_edited_draft_files option.

Add $resume_edited_draft_files option.

This adds an extra header when saving edited draft files (-E -H on the
command line).  With this header, the next time they are edited, they
are automatically "resumed" (by setting $resume_draft_files).

The idea is to prevent multiple user-defined headers and signatures
from being added to the draft message by avoiding processing it as a
brand new message after the first time.