Avoid crashing on new lines in /etc/fstab

Also handle /dev/shm mounts, which are not shown in the output of /etc/mounts. (/etc/mtab, however, records the /dev/shm mounts)
check-fstab-mounts.rb reads from /etc/mtab now
Merge pull request #613 from greezybacon/patch-1

Avoid crashing on new lines in /etc/fstab