magit-annex-dropunused: Fix '--force all'

The previous structure of the '--force all' arguments used "all" as the
cdr, which led to an error in the `mapconcat' call of
`magit-process-setup'.

    magit-insert-heading: Wrong type argument: listp, "all"
magit-annex-log-unused: Update argument name

See Magit 70e46617b2735f38fc9d827c19da928eac1d7705.
magit-annex-log-unused: Fall back on main log
magit-annex-log-unused: Use '--stat' option

With '--stat', the name of the symlink for the annex data will be
displayed, making it easier to know what the underlying data was hooked
up to.
Update argument processing for popup commands

Magit popups now define a function PREFIX-arguments [1]. Change popup
commands to use this instead of `magit-current-popup-args'.

Also, allow all arguments for popup commands to be specified when
directly calling the function.

[1] 56fc753b65cf008d85156e53c0a51d43e908a6c1