timeout(1): align execvp(3) failure statuses with GNU timeout

timeout(1): align execvp(3) failure statuses with GNU timeout

Align our exit statuses with those of GNU timeout in the execvp(3)
failure case.  Exit with 127 if the utility is not found.  Exit with
126 if we cannot execute the utility for any other reason.

While here, the child should _exit(2) instead of calling exit(3) via
err(3).

Update the manpage accordingly.

With input from millert@ and deraadt@.

Link: https://marc.info/?l=openbsd-tech&m=169739592322978&w=2
ok millert@