libdtrace: Be less strict when comparing pointer types

If one of two pointers refers to a forward declaration, let the pointers
be compatible so long as the referred types have the same name.
Otherwise we can get spurious errors.

To give a specific example, this can happen when ipfw_nat.ko is loaded
before ipfw.ko and /usr/lib/dtrace/ipfw.d is processed. Currently,
ipfw_nat.ko does not have a definition for struct inpcb (i.e., none of
its files include in_pcb.h), so in the CTF type graph, struct
ip_fw_args' "inp" member refers to a forward declaration, represented in
CTF with CTF_K_FORWARD.

Then, when libdtrace processes the ipfw_match_info_t translator in
ipfw.d, it decides that the "inp" field assignment is incorrect because
the two pointers are incompatible. However, there's no harm in allowing
this assignment. Add some logic to dt_node_is_ptrcompat() to detect
this case and declare the pointers as compatible so long as the name of
the thing they refer to is the same, similar to how any pointer is
compatible with a void *.

Reported by: marck
Reviewed by: Domagoj Stolfa <domagoj.stolfa@gmail.com>
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D48254