formpost: support quotes, commas and semicolon in file names

- document the double-quote and backslash need be escaped if quoting.
- libcurl formdata escape double-quote in filename by backslash.
- curl formparse can parse filename both contains '"' and ',' or ';'.
- curl now can uploading file with ',' or ';' in filename.

Bug: http://curl.haxx.se/bug/view.cgi?id=1171