Gmail style attachment processing

Gmail style attachment processing

This patch adds:

 * the search terms "filename" and "filetype"
 * hidden/reserved attachment label to track attachments
 * new flag on thread index mode to denote an attachment '@'
 * a "has:attachment" search query.

You can now search for something like "from:phil* filetype:pdf" for all
messages from a person called phil with a pdf attachment. You can also
specify a file name for the attachment with "filename:(this is a
filename with spaces.txt)". You can use wildcards in the filename
("filename:test*.pdf"). You can search for all messages from someone
with attachments using "from:someone has:attachment"

It will require sup-sync -all to work against all sources, but attempts
to be backwards compatable with non-synced sources.