Add TRIM support to clientside of frisbee MFS.
Currently this is done using the -E (erase) option to zapdisk to erase a partition or the entire disk. We run zapdisk before starting a frisbee load. Ultimately, I want TRIM support in frisbee proper (to erase rather than skip or zero free blocks), but erasing the entire disk might be a better option anyway (frisbee itself will benefit from the TRIMed disk). Notes: We only trim the disk when a node is in reloading. Didn't want to take a chance on having a user reload a partition on their disk and wind up getting the entire disk erased. That should not happen, but I didn't want to think through all the possibilities right now. Only works on FreeBSD right now. Linux MFS will still need to be modified. Just didn't want to track down how Linux does block erase right now. The clientside here is different than what is used on the Moonshots. On the Linux-based Moonshot MFS, we TRIM in slicefix by running fstrim on the root FS we just layed down. This used to be a good alternative back when the root FS occupied the entire disk, but now it won't work as well as we will only trim blocks from the first 6GB. Need to switch this over to the zapdisk (or frisbee) based TRIM. The server infrastructure on the Moonshots is the same: tmcc passes a TRIM=1 flags in "loadinfo" when the disk should be trimmed. So when to TRIM is based on the time since the last TRIM, and can now be specified site-wide, per-nodetype, or per-node. Currently, slicefix (x86 or ARM) doesn't do anything to the FS layed down to ensure that TRIM is done during the lifetime of the FS. Both FreeBSD and Linux have such options that can be set when the FS is created or added with tunefs. We need to think about this for mkextrafs and local blockstores as well.