gc.c refactoring

The attached patch passes all tests in gmake check.
Its intent is to clarify the code a little, and reduce repetition.

 * The "magic number" 1.8 gets a #define.
 * A for(;;) with break and continue is replaced by a smaller while loop.
 * The repeated case where objects are tested in 3 ways to see if they
   should not be marked is taken out to a function.

Unfortunately, none of this helps with the memory leak I've been
investigating.

It is offered in the hope it of some use.

I have found out over the weekend that realloc should act as malloc when
given a null pointer, according to ANSI C.  The 'if' test I originally
wrote about could possibly be avoided if the heap is passed to realloc
whether or not the heap size is zero, provided the heap pointer is null
at initialization and whenever heap size gets to zero.  However, given
the backwards compatibility issues this could raise on older systems, I 
have not implemented this.  Maybe someone with more experience of portability
issues may like to examine this?

        Thank you,
        Hugh