When copying types from one CTF container to another, ensure that we
encode 0-length (i.e. "") structure and union member names as offset 0.
This ensures that we don't confuse other parts of the CTF code which
expect this encoding.

This resolves a Dtrace error resolving members of anonymous structs/unions
within the (struct mbuf) type which some users were seeing after r366908.

While here, update the code in ctf_add_generic() to encode 0-length type
names as offset 0.

Reviewed by: markj
MFC after: 2 weeks
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D27246