Remove the 4.3BSD compatible macro m_copy(), use m_copym() instead

[kevlo]

Summary

It doesn't make sense to keep the m_copy() macro which was added for
compatibility with 4.3BSD, should always use m_copym() instead.

[rwatson]

Should we also be ditching M_COPY() and/or switching it to M_COPYM() for consistency..?

[kevlo]

Thanks for pointing that out. Here's the revised patch that switches
M_COPY() to M_COPYM().

[kevlo]

Committed in r305824.