multi: make the "general" list of easy handles a Curl_llist

Instead of having an especially "unique" linked list handler for the
main list of easy handles within the multi handle, this now uses a
regular Curl_llist for this as well.

With this change, it is also clearer that every easy handle added to a
multi handle belongs to one and only one out of three different lists:

 process - the general one for normal transfer processing

 pending - queued up waiting to get a connection (MSTATE_PENDING)

 msgsent - transfer completed (MSTATE_MSGSENT)

An easy handle must therefore be removed from the current list before it
gets added to another.

Closes #14474