lib: save a bit of space with some structure packing

- Reorder some internal struct members so that less padding is used.

This is an attempt at saving a bit of space by packing some structs
(using pahole to find the holes) where it might make sense to do
so without losing readability.

I.e., I tried to avoid separating fields that seem grouped
together (like the cwd... fields in struct ftp_conn for instance).
Also abstained from touching fields behind conditional macros as
that quickly can get complicated.

Closes https://github.com/curl/curl/pull/6483

lib/multi.c

@@ -200,8 +200,8 @@ struct Curl_sh_entry {
   struct Curl_hash transfers; /* hash of transfers using this socket */
   unsigned int action;  /* what combined action READ/WRITE this socket waits
                            for */
-  void *socketp; /* settable by users with curl_multi_assign() */
   unsigned int users; /* number of transfers using this */
+  void *socketp; /* settable by users with curl_multi_assign() */
   unsigned int readers; /* this many transfers want to read */
   unsigned int writers; /* this many transfers want to write */
 };