px4_posix: add px4_exit()

[TSC21]

Describe problem solved by the proposed pull request
While doing some adaptations on the uavcan driver, I faced that, for a POSIX target, one bumps into error: attempt to use poisoned "exit".

Test data / coverage
Tested under SITL.

Describe your preferred solution
This solution creates a px4_exit(status) under px4_posix.h, replacing the exit() call with a return call.

Additional context
I don't have any clear usage on the current source code, but maybe there is. @julianoes @dagar maybe you can point out some places where we can use this as a replacement to the exit calls.

TODO

• Add usage?

[julianoes]

I think this is fine for now but it won't work in general because it depends on the function having return type int (and not void).

[TSC21]

I think this is fine for now but it won't work in general because it depends on the function having return type int (and not void).

I guess I can make it more general in that sense.

[bkueng]

Can you look into where and why uavcan is using it?

[TSC21]

Can you look into where and why uavcan is using it?

Yest I can and I can replace it on a PR.