Default to building MCA components into the library, rather than
as dso objects, since most users are not taking advantage of
per-library packaging, and this results in less stat/open/etc. calls
during application startup.

Clean up the static/dso selection logic for componets at the same time.
Before, it was relatively hard to describe what would happen if
there was a mix of --enable-mca-dso and --enable-mca-static arguments.
Now, the code will first look for a component-specific argument, then
a framework-level argument, then a global argument.  If there is a tie,
static is preferred.

Update the NEWS and README.md to document the changes.  Jeff Squyres
wrote the original update to the README file, which was adopted to
the new README.md format.

Signed-off-by: Brian Barrett <bbarrett@amazon.com>