When copying a type from a source CTF container to a destination container,
ctf_add_type() first performs a by-name lookup of the type in the
destination container. If this lookup returns a forward declaration for an
enum, struct, or union, reset dst_type back to CTF_ERR, indicating that the
source type is not in fact present in the destination container. This
ensures that ctf_add_type() will also search the destination container's
dynamic type list for the source type.

Without this change, a pair of mutually recursive struct definitions could
cause infinite recursion in ctf_add_type() if the destination container
only contained forward declarations for the struct types: ctf_add_type()
recursively calls itself on each struct member's type, and the forward
declarations meant that the dynamic type list search would be skipped.

MFC after: 2 weeks
Sponsored by: EMC / Isilon Storage Division