trim /cache partition on suspend...

...if the partition has not been trimmed for more than 15 minutes
and no media playback is happening.

Requires fstrim binary and suspend_trim helper script in /sbin
directory (cf. commit 'Fancy Kernel r29' in fancykernel-RAMDISK
branch).
fs/suspend_trim.c: wait for the process to finish

Use UMH_WAIT_PROC rather than UMH_WAIT_EXEC, because sometimes
the fstrim process was frozen without being finished.
3.0.82