Fix H5Ovisit on files/groups, dangling links in link tests

[jwsblokland]

• Fixed the function RV_object_specific() for the case H5Ovisit. If the variable iter_object_type is either H5I_FILE or H5I_GROUP a call to CURL_PERFORM is made before setting the cURL headers correctly. This resulted in a segmentation fault. This has been fixed.
• Added the missing brackets in 2 if statements to ensure that the variable url_len is set first before the comparison evaluation.
• Corrected the link target path for the test_copy_link and test_move_link tests.
• Implemented a test for H5Ovisit() using file_id as root.