Renamed install/GNUmakefile to install-sbbs.mk

Made 'install' the default target (no longer need to include "target"
on installation command-line). Specify "build" if you want to perform
global build, only (no installation).

This will help resolve confusion and mistake when running 'make install'
in the wrong directory. Also, the installation command-lines and the
re-build command-lines will no be more different and less error-prone.

All documentation that references [install/]GNUmakefile now needs
updating <sigh>.