always-multi: always use non-blocking internals

Remove internal separated behavior of the easy vs multi intercace.
curl_easy_perform() is now using the multi interface itself.

Several minor multi interface quirks and bugs have been fixed in the
process.

Much help with debugging this has been provided by: Yang Tse

tests/data/test1208

@@ -36,6 +36,11 @@ FTP PORT download, no data conn and no transient negative reply
 <strippart>
 s/^EPRT \|1\|(.*)/EPRT \|1\|/
 </strippart>
+
+# This test doesn't send a QUIT because the main state machine in multi.c
+# triggers the timeout and sets the CURLE_OPERATION_TIMEDOUT error (28) for
+# which the FTP disconect code generically has to assume could mean the
+# control the connection and thus it cannot send any command.
 <protocol>
 USER anonymous
 PASS ftp@example.com
@@ -44,10 +49,9 @@ EPRT |1|
 TYPE I
 SIZE 1208
 RETR 1208
-QUIT
 </protocol>
 <errorcode>
-12
+28
 </errorcode>
 </verify>
 </testcase>