Fix test-e87b34ae: don't assume so much processes can be spawned at once.

The problem was reproducible this way:

    $ ulimit -u 800
    $ make -C tests
    ...
      CHECK test-e87b34ae FAILED
    ...

Reported-by: Jerome AUDU <jerome.audu@st.com>
Fix support for bindings where the guest path is explicitly not dereferenced.

I wonder how it may have worked before!  Warning: the syntax has
changed:

    before$ proot -b /bin/bash:!/bin/sh

    now$ proot -b /bin/bash:/bin/sh!
Prepare release v3.0.1.