Add code to dump part of a record as ASCII, and use it to dump strings

Add code to dump part of a record as ASCII, and use it to dump strings
in a source description record, including the stack.  Dump some other
fields in those records as well.

Attach separate sequential and random read buffers to the private data
structure, rather than allocating them in various routines (and not
always freeing them) and, in at least one case, allocating a single
*common* buffer for all wth's to use.

Fix some comments (the DS0 mask is 32 bytes long, but gets turned into a
bitmask).

Put in a description of what a "stack file"'s contents look like.  Much
of it may be useless to us (for example, we have the notion that TCP has
protocol number 6 built-in...), but the RELATION entries that map from
"BASE" to a protocol could obviate the need to have the user specify a
map from stack file names to starting protocols, and we might be able to
use, for example, entries that map TCP/UDP/SCTP port numbers to
protocols to obviate the need for the user to explicitly use Decode As
or otherwise configure port-to-protocol mappings themselves.

Add a bunch of record length checks before we fetch data from records.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@53450 f5534014-38df-0310-8fa8-9805f1628bb7