Optionally warn about missing zero blocks at the end of the archive

(In response to savannah bug #63574)

* doc/intern.texi: Document actual tar behaviour in regard to
missing end-of-file marker.
* doc/tar.texi: Rewrite the "warnings" section.  Document
--warning=missing-zero-blocks
* src/common.h (WARN_MISSING_ZERO_BLOCKS): New constant.
(WARN_ALL): Include all warning bits.
* src/list.c (read_and): If EOF is reached without seeing end-of-file
blocks and the "missing-zero-blocks" warning is requested, warn about
the fact.
* src/warning.c: New warnings: "missing-zero-blocks", "verbose".
(warning_option): Change definition to reflect changes in common.h

src/list.c

@@ -256,6 +256,13 @@ read_and (void (*do_something) (void))
 	  continue;
 
 	case HEADER_END_OF_FILE:
+	  if (!ignore_zeros_option)
+	    {
+	      char buf[UINTMAX_STRSIZE_BOUND];
+	      WARNOPT (WARN_MISSING_ZERO_BLOCKS,
+		       (0, 0, _("Terminating zero blocks missing at %s"),
+			STRINGIFY_BIGINT (current_block_ordinal (), buf)));
+	    }
 	  if (block_number_option)
 	    {
 	      char buf[UINTMAX_STRSIZE_BOUND];