mbuf(9), mbuf_tags(9): fix function prototypes.

• Add m_getclr(9) symlink to ObsoleteFiles.inc (removed in r295481).
• Add const qualifiers in m_dup(), m_dup_pkthdr() and m_tag_copy_chain()

(r286450).

• Fix m_dup_pkthdr() definition (it's not the same as m_move_pkthdr()).

MFC after: 5 days