libtar: Various bug fixes and enhancements

 * Fix compiler warnings
 * Fix memory leaks
 * Fix potential buffer overflows
 * Fix selinux context for symlinks
 * Fix off-by-one in gnu/ustar filename storage
 * Cleanup dead code
 * Add support for numeric-only uid/gid

Change-Id: If4daa7222b5c4f0e0741e7a3003fbf98fd60a872