Change to behaviour of "$ muddle" in src/

I have just changed muddle so that the behaviour of a "bare" muddle
anywhere under src/ has changed.


Before, if you were in src/, just typing "muddle" at the prompt would
do a REBUILD of all the checkouts "below" you (if you were in a
specific checkout source directory, then just of that checkout).
Rebuild unsets all the "built" flags, so has to look at each package
in turn and run its Makefile.

Now, that same "muddle" does a BUILD instead.

This means that if you are in src/ and do "muddle", and one of the
packages doesn't build, so you fix it, and then type "muddle" in the
same place, the building will resume from where it stopped, rather than
having to rebuild all the packages that had already built.

We believe this is an improvement. Let me know if it is not.

Other uses of a "bare" muddle continue the same as before - see

  $ muddle help label absent

(I split the muddle help for labels up into more manageable chunks).

Just for safety, the git repository before this change is tagged
"muddle-does-rebuild", and the commit for this change is tagged
"muddle-does_build-in-src" (the "_" in there is a typo, sorry).

Tibs