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/valgrind.supp

@@ -0,0 +1,16 @@
+{
+   libidn-idna_to_ascii-error
+   Memcheck:Addr4
+   fun:idna_to_ascii_4z
+   fun:idna_to_ascii_8z
+   fun:idna_to_ascii_lz
+   fun:fix_hostname
+   fun:resolve_server
+   fun:create_conn
+   fun:Curl_connect
+   fun:multi_runsingle
+   fun:curl_multi_perform
+   fun:curl_easy_perform
+   fun:operate
+   fun:main
+}