Add `DEFAULT_PATH` to match de-facto Linux standards

[TheDcoder]

DEFAULT_PATH replaces safepath for setting the PATH variable in the executed process's environment.

DEFAULT_PATH follows the de-facto standard of Linux distributions which place /usr/local directories before their non-local counterparts in $PATH.

Unlike BSD, Linux distributions don't put packaged executables under /usr/local, instead it is used by the local user to place their own executables, potentially to replace system executables.

This fixes #117