Annoyingly, for JSON formatting protojson randomly selects a spacing
method (one space or two spaces) depending on a hash of some sections of
the Go binary, to discourage depending on its output being stable.  This
breaks some checks in the CLI tests of the contents of fscrypt.conf and
the output of 'fscrypt status'.  As there doesn't appear to be a
straightforward alternative currently, for now just update the tests to
take into consideration the possible extra space.

World-writable directories are not appropriate for some systems, so
offer a choice of single-user-writable and world-writable modes, with
single-user-writable being the default.  Add a new documentation section
to help users decide which one to use.

Add a framework for writing automated tests of the fscrypt command-line
tool.  See cli-tests/README.md for details.