Completion for mutt's -a command line switch

[martin f krafft <madduck@debian.org>]

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Hey,

mutt allows attaching files from the command-line:

```
mutt -a /file/one /file/two /file/three -- …
```

Basically, the rules are: `-a` takes a list of files, terminated by 
`--`.

Zsh's completion of mutt treats the argument to `-a` as optional:

```
   '*-a[attach file using MIME]::file attachment:_files' \
```

Therefore, after `-a`, mutt's completions include recipients, which 
is not correct:

```
albatross:~% mutt -a toni^D
directory
toni/
recipient
toni@…
```

Could you please help me figure out how to fix this? Basically after 
it sees `-a`, compsys should be offering `_files` until `--` is 
encountered, and also: at least one file is mandatory.

Thanks,

-- 
  .''`.   martin f. krafft <madduck@d.o> @martinkrafft
: :'  :  proud Debian developer
`. `'`   http://people.debian.org/~madduck
   `-  Debian - when you have better things to do than fixing systems
  
"he gave me his card
  he said, 'call me if they die'
  i shook his hand and said goodbye
  ran out to the street
  when a bowling ball came down the road
  and knocked me off my feet"
                                                         -- bob dylan

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