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/test1390

@@ -80,7 +80,6 @@ MOOOO
 213 214
 150 Binary data connection for 1390 () (214 bytes).
 226 File transfer complete
-221 bye bye baby
 </file2>
 <stripfile2>
 s/^(229 Entering Passive Mode \().*(\).*)/${1}stripped${2}/