Add a 'statement of intent' blurb to the top of MAINTAINERS. Split

Add a 'statement of intent' blurb to the top of MAINTAINERS.  Split
the role of MAINTAINERS into advisory and strict parts. Introduce a
new LOCKS file to document enforced locked parts of the tree.

Strict locks are only added with core approval and will generally
have a renewal timeout.

Clarify that the source tree is a community effort, not a place to stake
out 'turf'.

This will be refined as needed.

With-core-hat-on:  Yes