Move connection.post_fd to http_post.post_fd.
Make connection.done point to the new done_http_connection(),
which calls the new done_http_post(), which closes post_fd.
So done_connection() no longer needs to do that.
Now that done_http_post() exists, a later commit can add dynamically
allocated data in struct http_post and ensure that it will be freed.
CGI scripts are distinguishable from normal files. I hope that this
fixes the bug 991. This commit also reverts the previous revert.
(cherry picked from commit 7ceba1e461)
Revert part of commit 7215c964e40afe953787d7831b04182fbaba4662,
"Use real types (enum connection_{state,priority})." of 2005-06-14.
connection.pri[] is indexed by enum connection_priority, but its
elements are merely reference counts; they are never assigned from
or compared to enum connection_priority. Defining the elements
as int will result in more readable output from GDB.
Noted in bug 920.
This simplifies unqueuing of downloads and makes it more obvious that
the 'change' being performed is to migrate or replace an old download
handle with a new one.
