Because the `strerror` result doesn't include a newline, we need to add
one. Observed on a minimal system that doesn't have `man` installed,
which behaves like this before the fix:
```
[root@upper tim]# zpool help import
couldn't run man program: No such file or directory[root@upper tim]#
```
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Hatch <tim@timhatch.com>
Closes#18183