Provide search and filter with the option to specify more flag based
conditions.
Use '-x <flag>' to search for messages with a flag (seen, answered,
flagged) and '-X <flag>' to search for messages without a flag.
This changes the search flags for maildir and imap backends.
They now no longer use -t for searching all text. This seems to make
more sense as being the targeted recipient. I have similarly added Cc
for -c. The text search now resides under -a for all text.
