Changes to :write-content-to-file parameter of PARSE-MIME

Previously only values NIL and T were accepted, where T meant that
content of a MIME part is written to a file instead of returned as a
string.

This change also allows to give a function, which is called with MIME
part's headers, and should return either a pathname of a file where
the MIME part should be written, or an open stream [to a file] where
the MIME part is to be written.
Improved Content-Type header treatment

Mainly by removing lame code.  Best way of improvement.
PARSE-MIME callback function called with useful parameters

Now the callback function will be called with the following
self-documenting keyword parameters:

 :field-name
 :file-name
 :content-type

Also, the function will be called with :allow-other-keys t, so user
does not have to specify &allow-other-keys if some of the parameters
are not used.
Drop empty file upload fields