curl_formadd: fix file upload on VMS

For the standard VMS text file formats, VMS needs to read the file to
get the actual file size.

For the standard VMS binary file formats, VMS needs a special format of
fopen() call so that it stops reading at the logical end of file instead
of at the end of the blocks allocated to the file.

I structured the patch this way as I was not sure about changing the
structures or parameters to the routines, but would prefer to only call
the stat() function once and pass the information to where the fopen()
call is made.

Bug: https://sourceforge.net/p/curl/bugs/758/