Have _heapmaxavail() work as advertised.

The cc65 doc explicitly states that the return value of _heapmaxavail() can be used as parameter of malloc(). To actually allow for that the size of HEAP_ADMIN_SPACE has to be substracted from the raw size of the largest free heap block.

libsrc/common/_heapmaxavail.s

@@ -69,8 +69,17 @@ __heapmaxavail:
         stx     ptr1
         jmp     @L1
 
-; return Size;
+; if (Size < HEAP_ADMIN_SPACE) return 0;
 
 @L3:    lda     ptr2
+        sub     #HEAP_ADMIN_SPACE
         ldx     ptr2+1
+        bcs     @L5
+        bne     @L4
+        txa
         rts
+
+; return Size - HEAP_ADMIN_SPACE;
+
+@L4:    dex
+@L5:    rts