Change CurlMultiClient for libcurl3 7.32

Starting with libcurl3 7.32, curl_multi_select often returns -1 immediately,
and continues to do so until curl_multi_exec is called to perform more work.

See these PHP bugs:
https://bugs.php.net/bug.php?id=63411
https://bugs.php.net/bug.php?id=65704

This patch simplifies the processing loop, and makes sure that curl_multi_exec
is called even when curl_multi_select returns -1. It has been tested with
libcurl3 7.26 and 7.32, both using PHP 5.5.4 on Debian.

Change-Id: I996d803d25d1093c68ab21f4b37e88efa24ad277
Merge "Change CurlMultiClient for libcurl3 7.32"