fspacectl meets DIOCGDELETEE

fspacectl(2) does for regular files the same thing that DIOCGDELETE
does for GEOM devices.  The only differences are that DIOCGDELETE
requires the operation to be block-aligned, and if interrupted
DIOCGDELETE doesn't give feedback about partial progress.  Can we
connect the two?  That would allow a user to free space with a single
API for both files and devices.  It would require:

* Adding a d_fspacectl_t member to struct cdevsw
* Implementing d_fspacectl_t for g_dev_cdevsw by using DIOCGDELETE
* Implementing .fo_fspacectl for devfs
* Optionally implementing d_fspacectl on other device types, like zvols.

-Alan