Errored: OpenSmalltalk/opensmalltalk-vm#1349 (Cog - 3487eed)

CogVM source as per VMMaker.oscog-eem.2394

Sista V1 Bytecode Set/Full Blocks:
Fixed a bug in frameless full blocks (fetching receiver from receiver index in
FullBlock and not outerContext)

SpurSelectiveCompactor:
Add a guard to followForwardingPointersInStackZone: to avoid scanning the
stack zone when it is empty because the system is snapshotting.

Added segmentOverlap assertions and debugging convenience.

Add optional read barrier for temp vector accesses
(TempVectReadBarrier Slang to C compilation settings)

Plugins General:
Added #methodReturnReceiver, another methodReturn* method, that pops
argumentCount elements from the stack, leaving only the receiver there

BitBltPlugin:
Don't fail primitiveDisplayString for empty strings (still validates fully).

FilePlugin:
Use methodReturn***: instead of pop: + push***:.
Change the primitiveFileDescriptorType to return the file descriptor type
instead of just 0 or 1
Eliminate obsolete accessor (to get rid of a cCode:inSmalltalk:).

MiscPrimitivePlugin:
- somewhat decreased the overhead of the primitive calls by trying to minimize
interpreterProxy function calls
- primitiveFindFirstInString:
- eliminated the start variable
- fail with PrimErrBadIndex if the start index argument is invalid (<= 0)
- primitiveFindSubstring
- fail with PrimErrBadArgument if matchTable is too short
- primitiveIndexOfAsciiInString
- fail with PrimErrBadIndex if the start index argument is invalid (<= 0)
- primitiveStringHash
- eliminated the byteArraySize variable
- moved the masking out of the loop, so it's only evaluated once
- primitiveTranslateStringWithTable
- fail with PrimErrBadIndex if the start or stop index argument is invalid
(<= 0, >= size respectively)
- primitiveConvert8BitSigned
- added primitive error codes

Slang:
Trust config.h more. It declares when to include dlfcn and takes care of
_GNU_SOURCE.