Return -EACCESS when execve'ing a directory, as expected by some programs.

The problem was exposed by "env", see test-pppppppp.sh for details.
Fix the search of the initial command, performed by the built-in which().

When the initial command is a symbolic link, PRoot has to dereference
it in guest namespace, not in the host one.

Reported-by: Mickael GUENE <mickael.guene@st.com>
Prepare release v3.0.2.