Pike.gc_parameters(): Added some gc callbacks.

Adds callbacks that are called from the gc to simplify
debugging of memory leaks and similar.
Pike.gc_parameters: The pre_cb now actually works.

Fixes infinite recursion when a pre_cb is installed and gc()
is run implicitly.
Tools.AutoDoc: Improved parsing of implicit create().

Support doc following the variable declaration in implicit create().

Fixed some double spaces in error messages.

Removed unused variable.
Runtime: Improve robustness against outputting wide strings.

master()->handle_error() now survives if eg an object has
an _sprintf() that returns a wide string.

master()->compile_{error,warning}() now survive messages about
wide symbols.

Made a similar fix in Hilfe to survive compiler warnings and
errors about wide symbols.

Fixes [bug 6805].
Stdio.UDP: Support IPv6 in {add,drop}_membership()

FIXME: The second argument (interface address) is not supported yet.
SNMP: Fixed error in GetRequest variable bindings.

The variable value should be ASN1 "Null" rather than a bogus integer.
C.f. RFC 1905 section #3.
pike_memory: replaced old block allocator
Compiler: No need for __INIT() to clear variables.

Compat: This changes the semantics of calling __INIT()
        by hand slightly.
_Ffmpeg: Support CODEC_IDs prefixed with AV_.

Fixes some compilation issues with more modern versions of ffmpeg.
_Ffmpeg: Survive recent versions of AVCodec.

The field "encode" seems to have been renamed to "encode2"
in current versions of libavcodec.
Added dumping support. Removed unfinished quoting.
Some more URI tests.
Compiler: Fixed NULL-deref by docode().

Call fix_type_field() before calling docode() in eval_low() to
ensure that the parsetree node types have been properly set.

This fixes a NULL-deref in docode() when compiling the line:

  string system_module_path=master()->system_module_path[-1];

(The F_ARROW node in the rvalue had a NULL type).
docode() requires that the parsetree node types h

This fixes a NULL-deref in docode() when compiling the line
(from lib/modules/Tools.pmod/Standalone.pmod/module.pmod:28):

  string system_module_path=master()->system_module_path[-1];

(The F_ARROW node in the rvalue had a NULL type).
block_alloc: possible use after free
_Ffmpeg: Removed redundant calls to avcodec_init().

avcodec_init() is called from avcodec_register_all(), and
has been removed from recent versions of libavcodec.
_Ffmpeg: Checks for deprecated functions.

avcodec_decode_audio3() and avcodec_open() are marked
as deprecated in current (0.10.3) ffmpeg. This adds
configure checks for the replacements, and some FIXMEs.
_Ffmpeg: Avoid duplicate CODEC_ID_* constants.
lower_mega_apply: fix "bad offset"
added valgrind macros for pike_frame and catch_context structs
Stdio.UDP: Support IPv6 multicast.

enable_multicast() and set_multicast_ttl() now support IPv6.

Also improves documentation for the multicast-related functions.