BibUpload: os.path.realpath to os.path.abspath

• BibUpload and BibDocFile libraries were using os.path.realpath to obtain an absolute path for inputted files. This was also used for security checking. However, os.path.realpath resolves symbolic links, thus some good absolute path containing a symbolic link would have been resolved to a different path. This is now solved by using abspath in place of realpath.