More mail flags can now be set, unset, and toggled, not just the
read/seen flag.
This functionality is implemented with a new `:flag` and `:unflag`
command, which are extensions to the matching `:read` and `:unread`
commands, adding support for different flags. In fact, the
`read`/`unread` commands are now recognized aliases to `flag`/`unflag`.
The new commands are also well documented in aerc(1).
The change mostly extends the previous read/unread setting functionality
by adding a selection for the flag to change.
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