initscript: run fstrim at boot time

This will prevent severe slow downs after months and months of usage at the cost of maybe a slower boot time (assuming the network would be ready that fast).

This option was preferred over adding -o discard when mounting the drive because of performance reasons: we do not need to discard every block immediately, once at boot time provides pretty much 100% of the benefits (not hitting a performance wall after some time) at no runtime cost at all.

This command is allowed to fail.

Closes: #36 (closed) Signed-off-by: Martin Roukala (né Peres) martin.roukala@mupuf.org