Support application/news-groupinfo entity in controlchan

[Julien ÉLIE]

controlchan should be able to properly decode newgroup control articles which contain an application/news-groupinfo entity. It is a new structure for such articles described in USEPRO.

[Julien ÉLIE]

(In [8281]) Use MIME::Parser in controlchan so that we could decode MIME
control headers (especially those which contain application/news-groupinfo
entities, as described in USEPRO).

Also fix a long-standing bug of uninitialized variables when
some headers were missing.

Update INSTALL to mention the dependency on MIME::Parser.

close #122

[Julien ÉLIE]

(In [8282]) Add a warning at configure time if MIME::Parser or Encode
are missing.

see #122

[Julien ÉLIE]

(In [8283]) Do not output temp to core because it can lead to memory issues
(or attacks/starvation). Instead, use temporary files, which
is incidentally the default for MIME::Parser.
controlchan is spawn during a long time, so it is necessary.
We purge the temporary files once used.

see #122