bug#17713: First Portion of 'backtrace`

[Geoff Shannon]

(gdb) backtrace
#0  0x0000000000542e54 in mark_object (arg=<optimized out>) at alloc.c:5979
#1  0x00000000005438ae in mark_vectorlike (ptr=0x60e8a68) at alloc.c:5842
#2  0x00000000005430f2 in mark_object (arg=<optimized out>) at alloc.c:6174
#3  0x0000000000543109 in mark_object (arg=<optimized out>) at alloc.c:6301
#4  0x000000000054317c in mark_object (arg=<optimized out>) at alloc.c:6192
#5  0x00000000005438ae in mark_vectorlike (ptr=0x11e0130) at alloc.c:5842
#6  0x00000000005430f2 in mark_object (arg=<optimized out>) at alloc.c:6174
#7  0x000000000054302c in mark_object (arg=<optimized out>) at alloc.c:6188
#8  0x00000000005438ae in mark_vectorlike (ptr=0x32eec98) at alloc.c:5842
#9  0x00000000005430f2 in mark_object (arg=<optimized out>) at alloc.c:6174
#10 0x000000000054302c in mark_object (arg=<optimized out>) at alloc.c:6188
#11 0x0000000000543109 in mark_object (arg=<optimized out>) at alloc.c:6301
#12 0x00000000005432a4 in mark_object (arg=<optimized out>) at alloc.c:6088
#13 0x000000000054302c in mark_object (arg=<optimized out>) at alloc.c:6188
#14 0x0000000000543109 in mark_object (arg=<optimized out>) at alloc.c:6301
#15 0x0000000000543109 in mark_object (arg=<optimized out>) at alloc.c:6301
#16 0x0000000000543035 in mark_object (arg=<optimized out>) at alloc.c:6189
#17 0x00000000005432a4 in mark_object (arg=<optimized out>) at alloc.c:6088
#18 0x000000000054302c in mark_object (arg=<optimized out>) at alloc.c:6188
#19 0x000000000054317c in mark_object (arg=<optimized out>) at alloc.c:6192
#20 0x00000000005438ae in mark_vectorlike (ptr=0x4ffd358) at alloc.c:5842
#21 0x00000000005430f2 in mark_object (arg=<optimized out>) at alloc.c:6174
#22 0x000000000054302c in mark_object (arg=<optimized out>) at alloc.c:6188