mk: Don't rm 'dist' during clean, just its contents

This is not for temporaries now
mk: Fix a minor UI bug
mk: Fix prepare.mk

The way it was formulated you could only 'prepare' one directory per build.
Revert "mk: Run 'make install' through install.sh"

This reverts commit e93709a911637194835268420e67d768ee19b5df.
mk: Make distcheck depend on dist-docs