logging: Use lg_exitstatus

Replace two open-coded exit status checks with calls to lg_exitstatus.

In the case of slip.c and udp.c this has no significant effect other
than a slight change to message format.

In the case of process.c, we no longer log the command's first
argument.  I consider this tolerable for simplifying the code.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>