mkarchiso: use mksquashfs -quiet instead of redirecting its stdout to /dev/null

mksquashfs supports a -quiet option since squashfs-tools 4.4.
Use this option in non-verbose mode instead of redirecting stdout of the whole command to /dev/null.

This allows to have only one instance of mksquashfs in _run_mksquashfs instead of multiple ones in if-then-else.

Related to #148.