lib544.c: use duphandle for test 545

To verify that curl_easy_duphandle() works fine on a handle that has
gotten data stored with *_COPYPOSTFIELDS.
curl_easy_duphandle: CURLOPT_COPYPOSTFIELDS read out of bounds

When duplicating a handle, the data to post was duplicated using
strdup() when it could be binary and contain zeroes and it was not even
zero terminated! This caused read out of bounds crashes/segfaults.

Since the lib/strdup.c file no longer is easily shared with the curl
tool with this change, it now uses its own version instead.

Bug: http://curl.haxx.se/docs/adv_20141105.html
CVE: CVE-2014-3707
Reported-By: Symeon Paraschoudis
RELEASE-NOTES: 7.39.0 release (commit b3875606925)