fix: pass file permissions to open(2) in arch_open on POSIX

The POSIX arch_open macro passed the Windows-style share flag as the
mode argument of open(2) and ignored the actual permission argument.
Files created by the file tape backend got mode 0200 (write-only), so
a non-root user could not reopen records it had just written; mounting
a freshly formatted file-backend volume failed with EDEV_RW_PERM.
Running as root masked the problem.

Author: matejk

src/libltfs/arch/ltfs_arch_ops.h

@@ -151,7 +151,9 @@ extern "C" {
 
     #define arch_sscanf     sscanf
 
-    #define arch_open( descriptor_ptr, filename_ptr, open_flg, share_flg, unused) do{ *descriptor_ptr = open(filename_ptr, open_flg, share_flg); }while(0)
+    /* POSIX open(2) takes the permission mode as its third argument; the
+     * Windows-style share flag is not applicable here. */
+    #define arch_open( descriptor_ptr, filename_ptr, open_flg, share_flg, perm) do{ *descriptor_ptr = open(filename_ptr, open_flg, perm); }while(0)
 
     #define arch_fopen(file, mode, file_ptr)  do {file_ptr = fopen(file, mode);}while(0)