Fix a typo in a comment.
In m_megapullup() instead of reserving some space at the end of packet,
m_align() it, reserving space to prepend data.

Reviewed by:	mav
Add MEXT_ALIGN() macro, similar to M_ALIGN() and MH_ALIGN(), but for
mbufs with external buffer.
In m_align() add assertions that mbuf is virgin, similar to assertions
in M_ALIGN(), MH_ALIGN, MEXT_ALIGN() macros.