CMSG_DATA(3): Use consistent variable names

The description of CMSG_FIRSTHDR used two variables (mhdr and msg) to
reference the same thing. Use msghdr consistency across the manual page
instead.

Reviewed by: bcr
Approved by: krion (mentor, implicit), mat (mentor, implicit)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D17804