psh: exit with correct return code

Currently `psh` exits always with `0` which makes any of the applets unusable in complex environment.

Also: revisit psh applet return codes, as only the lower 8 bits are passed to the parent process - so returning negative values might not be the best idea.

For details see: https://pubs.opengroup.org/onlinepubs/007904875/functions/exit.html