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/test1351

@@ -55,6 +55,8 @@ fooo
 mooo
 </file1>
 
+# The final "221 bye bye baby" response to QUIT will not be recorded
+# since that is not considered part of this particular transfer!
 <file2 name="log/heads1351">
 220-        _   _ ____  _     
 220-    ___| | | |  _ \| |    
@@ -70,7 +72,6 @@ mooo
 213 10
 150 Binary data connection for 1351 () (10 bytes).
 226 File transfer complete
-221 bye bye baby
 </file2>
 <stripfile2>
 s/^(229 Entering Passive Mode \().*(\).*)/${1}stripped${2}/