formadd: CURLFORM_FILECONTENT wrongly rejected some option combos

The code for CURLFORM_FILECONTENT had its check for duplicate options
wrong so that it would reject CURLFORM_PTRNAME if used in combination
with it (but not CURLFORM_COPYNAME)! The flags field used for this
purpose cannot be interpreted that broadly.

Bug: http://curl.haxx.se/mail/lib-2013-07/0258.html
Reported-by: Byrial Jensen