file-pcapng: redo the way we dissect the data in blocks.

Create a tvbuff that covers the data portion of a block, and use that to
dissect all data in the block, including but not limited to the options.

Catch ReportedBoundsError exceptions and treat them as an indication
that the block length was too short - add an expert info to the block
length item indicating that.

Have separate routines for each block type that dissects the data in
that block type.

While we're at it, check whether the trailing block length is equal to
the  header block length and, if not, report an error in the trailing
block length.

Fix the tests to match.

test/suite_fileformats.py

@@ -243,6 +243,9 @@ class case_fileformat_mime(subprocesstest.SubprocessTestCase):
         proc = self.assertRun((cmd_tshark,
                 '-r', capture_file('icmp.pcapng.gz'),
                 '-Xread_format:MIME Files Format',
-                '-Tfields', '-e', 'frame.len', '-e', 'pcapng.block.length',
+                '-Tfields',
+                '-e', 'frame.len',
+                '-e', 'pcapng.block.length',
+                '-e', 'pcapng.block.length_trailer',
             ))
-        self.assertEqual(proc.stdout_str.strip(), '480\t128,128,88,88,132,132,132,132')
+        self.assertEqual(proc.stdout_str.strip(), '480\t128,88,132,132\t128,88,132,132')