Replace the introduction to the SMPng page, which largely dated from 2003,
with a new introduction that reflects the work that has gone into the 5.x
and 6.x branches. In particular, update the status to reflect MPSAFE
networking, MPSAFE VFS, MPSAFE VM, on-going performance work, and some of
the important steps to get there. Highlight the iterative nature of the
work, and the focus on infrastructure.