It confuses code analyzers with its use of -1 for unsigned value. Also,
a check that's not normally used in strdup() code - and not necessary.
Closes#5697
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
