(http://curl.haxx.se/bug/view.cgi?id=1431750) helped me identify and fix two
different but related bugs:

1) Removing an easy handle from a multi handle before the transfer is done
   could leave a connection in the connection cache for that handle that is
   in a state that isn't suitable for re-use. A subsequent re-use could then
   read from a NULL pointer and segfault.

2) When an easy handle was removed from the multi handle, there could be an
   outstanding c-ares DNS name resolve request. When the response arrived,
   it caused havoc since the connection struct it "belonged" to could've
   been freed already.

Now Curl_done() is called when an easy handle is removed from a multi handle
pre-maturely (that is, before the transfer was complteted). Curl_done() also
makes sure to cancel all (if any) outstanding c-ares requests.

type to the already provided type CURLPROXY_SOCKS4.
I added a --socks4 option that works like the current --socks5 option but
instead use the socks4 protocol.

content when libcurl didn't honor the internal ignorebody flag.

code. It should however not be the cause of any troubles. He also fixed a
few similar problems in the HTTP test server code.

as previously it could be holding on to old cached entries longer than
requested.

  server configured in the system, the ares_init() call fails and thus
  curl_easy_init() fails as well. This causes weird effects for people who use
  numerical IP addresses only.

an app can use to let libcurl only connect to a remote host and then extract
the socket from libcurl. libcurl will then not attempt to do any transfer at
all after the connect is done.

instead of LDFLAGS.

due to the use of the static variable for sigsetjmp()

with re-used FTP connections. If the second request on the same connection was
set not to fetch a "body", libcurl could get confused and consider it an
attempt to use a dead connection and would go acting mighty strange.

(occurs w/o ENABLE_IPV6).

curl tool with --local-port. Plain and simply set the range of ports to bind
the local end of connections to. Implemented on to popular demand.

Not extensively tested. Please let me know how it works.

connection setup as a follow-redirect. It turns out 1) this fails when a FTP
connection is re-setup and 2) it does make the max-redirs counter behave
wrong. This fix was not verified since the reporter vanished, but I believe
this is the right fix nonetheless.

of the system instead of depending on what particular systems we think need
various headers

fact equal to a blank one according to the spec.

even after EPSV returned a positive response code, if libcurl failed to
connect to the port number the EPSV response said. Obviously some people are
going through protocol-sensitive firewalls (or similar) that don't understand
EPSV and then they don't allow the second connection unless PASV was
used. This also called for a minor fix of test case 238.

Tried to make some of the wording a bit more consistent.

(CURLOPT_FTPPORT) didn't work for ipv6-enabed curls if the IP wasn't a
"native" IP while it works fine for ipv6-disabled builds!

In the process of fixing this, I removed the support for LPRT since I can't
think of many reasons to keep doing it and asking on the mailing list didn't
reveal anyone else that could either. The code that sends EPRT and PORT is
now also a lot simpler than before (IMHO).