Header import, and dumporiginal extensions.

Add ability to dump headers or mimetypes for original messages.
Add ability to dump all messages, instead of just hashes listed.
Add new command  cmd/headers to import all headers from messages in table
    original.  Utility can work incrementally or do fully table imports.
Moved 'fetch all original messages' code to db/util.go.  Added conditional
version given table name that will only return original messages that do not
    have matching hashes in the specified table.

pg/init-header.sql

@@ -0,0 +1,7 @@
+DROP TABLE header;
+
+CREATE TABLE header (
+    hash CHAR(40) REFERENCES original (hash),
+    name TEXT,
+    value TEXT
+);