Fixed buffer overflows in ls-tree.c

Fixed buffer overflows in ls-tree.c

As reported in GitHub issue #24, tree dumping mode can smash the stack
if the hierarchy of buses is too deep.

Increased line buffer size to 1024 and switched to use of snprintf
everywhere, so that in the worst case, the line is truncated.

As snprintf can be problematic on obscure platforms, I wrapped it
in tree_printf(), so that we can add #ifdefs should problems arise.