Fix dereference replacements for pointers to pointers

Currently, the bcc rewriter is unable to track external pointers if
there is more than a single level of indirection (e.g., pointer to
external pointer).  For example, in the following, the rewriter is
unable to detect that ptr2 doesn't need a call to bpf_probe_read,
only *ptr2 do.

int test(struct pt_regs *ctx, struct sock *sk) {
    struct sock *ptr1;
    struct sock **ptr2 = &ptr1;
    *ptr2 = sk;
    return ((struct sock *)(*ptr2))->sk_daddr;
}

This commit fixes this issue by tracking the levels of indirections
in addition to the variable declarations (identifies each variable).
When traversing dereferences, the level of indirections is used to
decide whether the base expression is an external pointer.  The level
of indirections is inherited when a pointer is assigned to a new
variable (assignments and function calls).