slattach: pass up exit status of shell command

slattach: pass up exit status of shell command

This is partially to fix a build warning:
	slattach.c: In function 'main':
	slattach.c:771:9: warning: ignoring return value of 'system',
		declared with attribute warn_unused_result [-Wunused-result]

And partially because I think it's the right thing to do.  Otherwise
there's no way for people to check the exit status of the command they
told slattach to run.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>