Add new option so umeq use PATH to search for target executable and handle shebang exe

[mickael-guene]

So this new option will have two effects:

• The first one is to use PATH environment variable to find target executable unless path is in absolute form.
• The second one is to allow to give a shell script (or other shebang file) as target executable.

Final goal is to improve docker user experience when using docker image that embed umeq.

For example if this option as '-1' form we can have such docker file:

ENTRYPOINT ["/usr/bin/umeq-arm64", "-execve", "-1"]
CMD ["bash"]

which allow:

docker run -i -t

but also

docker ls

or

docker my_script.sh