curl_easy_perform_ev: debug/test function

This function is meant to work *exactly* as curl_easy_perform() but will
use the event-based libcurl API internally instead of
curl_multi_perform(). To avoid relying on an actual event-based library
and to not use non-portable functions (like epoll or similar), there's a
rather inefficient emulation layer implemented on top of Curl_poll()
instead.

There's currently some convenience logging done in curl_easy_perform_ev
which helps when tracking down problems. They may be suitable to remove
or change once things seem to be fine enough.

curl has a new --test-event option when built with debug enabled that
then uses curl_easy_perform_ev() instead of curl_easy_perform(). If
built without debug, using --test-event will only output a warning
message.

NOTE: curl_easy_perform_ev() is not part if the public API on purpose.
It is only present in debug builds of libcurl and MUST NOT be considered
stable even then. Use it for libcurl-testing purposes only.

runtests.pl now features an -e command line option that makes it use
--test-event for all curl command line tests. The man page is updated.

src/tool_getparam.c

@@ -174,6 +174,7 @@ static const struct LongShort aliases[]= {
   {"$I", "post303",                  FALSE},
   {"$J", "metalink",                 FALSE},
   {"$K", "sasl-ir",                  FALSE},
+  {"$L", "test-event",               FALSE},
   {"0",  "http1.0",                  FALSE},
   {"1",  "tlsv1",                    FALSE},
   {"2",  "sslv2",                    FALSE},
@@ -961,6 +962,13 @@ ParameterError getparameter(char *flag,    /* f or -long-flag */
       case 'K': /* --sasl-ir */
         config->sasl_ir = toggle;
         break;
+      case 'L': /* --test-event */
+#ifdef CURLDEBUG
+        config->test_event_based = toggle;
+#else
+        warnf(config, "--test-event is ignored unless a debug build!\n");
+#endif
+        break;
       }
       break;
     case '#': /* --progress-bar */