gdb: New set/show max-value-size command.

gdb: New set/show max-value-size command.

For languages with dynamic types, an incorrect program, or uninitialised
variables within a program, could result in an incorrect, overly large
type being associated with a value.  Currently, attempting to print such
a variable will result in gdb trying to allocate an overly large buffer.

If this large memory allocation fails then the result can be gdb either
terminating, or (due to memory contention) becoming unresponsive for the
user.

A new user visible variable in gdb helps guard against such problems,
two new commands are available:

   set max-value-size
   show max-value-size

The 'max-value-size' is the maximum size of memory in bytes that gdb
will allocate for the contents of a value.  Any attempt to allocate a
value with a size greater than this will result in an error.  The
initial default for this limit is set at 64k, this is based on a similar
limit that exists within the ada specific code.

It is possible for the user to set max-value-size to unlimited, in which
case the old behaviour is restored.

gdb/ChangeLog:

	* value.c (max_value_size): New variable.
	(MIN_VALUE_FOR_MAX_VALUE_SIZE): New define.
	(show_max_value_size): New function.
	(check_type_length_before_alloc): New function.
	(allocate_value_contents): Call check_type_length_before_alloc.
	(set_value_enclosing_type): Likewise.
	(_initialize_values): Add set/show handler for max-value-size.
	* NEWS: Mention new set/show command.

gdb/doc/ChangeLog:

	* gdb.texinfo (Value Sizes): New section.
	(Data): Add the 'Value Sizes' node to the menu.

gdb/testsuite/ChangeLog:

	* gdb.base/max-value-size.c: New file.
	* gdb.base/max-value-size.exp: New file.
	* gdb.base/huge.exp: Disable max-value-size for this test.