shutdowns: split shutdown handling from connection pool

Further testing with timeouts in event based processing revealed that
our current shutdown handling in the connection pool was not clear
enough. Graceful shutdowns can only happen inside a multi handle and it
was confusing to track in the code which situation actually applies. It
seems better to split the shutdown handling off and have that code
always be part of a multi handle.

Add `cshutdn.[ch]` with its own struct to maintain connections being
shut down. A `cshutdn` always belongs to a multi handle and uses that
for socket/timeout monitoring.

The `cpool`, which can be part of a multi or share, either passes
connections to a `cshutdn` or terminates them with a one-time, best
effort.

Add an `admin` easy handle to each multi and share. This is used to
perform all maintenance operations where no "real" easy handle is
available. This solves the problem that the multi admin handle requires
some additional initialisation (e.g. timeout list).

The share needs its admin handle as it is often cleaned up when no other
transfer or multi handle exists any more. But we need a `data` in almost
every call.

Fix file:// handling of errors when adding a new connection to the pool.

Changes in `curl` itself:

- for parallel transfers, do not set a connection pool in the share,
  rely on the multi's connection pool instead. While not a requirement
  for the new `cshutdn` to work, this is

  a) helpful in testing to trigger graceful shutdowns
  b) a broader code coverage of libcurl via the curl tool

- on test_event with uv, cleanup the multi handle before returning from
  parallel_event(). The uv struct is on the stack, cleanup of the multi
  later will crash when it tries to register sockets. This is a "eat
  your own dogfood" related fix.

Closes curl#16508

Author: icing

lib/Makefile.inc

@@ -123,6 +123,7 @@ LIB_CFILES =         \
   cf-socket.c        \
   cfilters.c         \
   conncache.c        \
+  cshutdn.c          \
   connect.c          \
   content_encoding.c \
   cookie.c           \
@@ -260,6 +261,7 @@ LIB_HFILES =         \
   cf-socket.h        \
   cfilters.h         \
   conncache.h        \
+  cshutdn.h          \
   connect.h          \
   content_encoding.h \
   cookie.h           \

lib/cfilters.c

@@ -202,7 +202,7 @@ CURLcode Curl_conn_shutdown(struct Curl_easy *data, int sockindex, bool *done)
   if(!Curl_shutdown_started(data, sockindex)) {
     CURL_TRC_M(data, "shutdown start on%s connection",
                sockindex ? " secondary" : "");
-    Curl_shutdown_start(data, sockindex, &now);
+    Curl_shutdown_start(data, sockindex, 0, &now);
   }
   else {
     timeout_ms = Curl_shutdown_timeleft(data->conn, sockindex, &now);