LibArchive: Implement proper support for Tar file end markers

Previously this was handled implicitly, as our implementation of Tar
would just stop processing input as soon as it found something invalid.
However, since we now error out as soon as something is found to be
wrong, we require proper handling for zero blocks, which aren't actually
fatal.

Userland/Libraries/LibArchive/Tar.h

@@ -129,6 +129,7 @@ public:
     unsigned expected_checksum() const;
     void calculate_checksum();
 
+    bool is_zero_block() const;
     bool content_is_like_extended_header() const;
 
     void set_filename_and_prefix(StringView filename);