We want to use PATH from the current value of the environment, not
the initial value of PATH when the policy was opened.
This is a little different from how real execvp() works since we
use stat() instead of just execve().
It now takes an intercept fd as an optional argument instead of a
list of extra variables to add. This lets us check whether it is
already set to the expected value (and add it if not).
sudo_intercept.so now uses sudo_preload_dso() to make sure that
LD_PRELOAD and SUDO_INTERCEPT_FD are set properly before executing.
