Make setting the custom icon more robust

- update VolumeIcon.icns if it has changed

- if the FinderInfo indicating there is a custom icon is not set,
  then set it, regardless of whether we happen to be copying
  VolumeIcon.icns
Change 'zed' to exit on SIGTERM as expected by reboot. Remove signal to
ourselves, and return int to signal restart (/dev/zfs returning ENODEV).
Allow 'zed' to receive TERM and quit while waiting for kext to load.
recreated zvol-unmap topic, branched from master
cstyle fixes
only exception: 32 lines longer than 80 chars in zvol.c
minor fix
restore const to correct location
Force termination of zvol devices
Remove unused performRead and performWrite
cstyle fixes
./scripts/cstyle.sh -cp include/sys/zvol* module/zfs/zvol* shows zero issues

Additional style cleanups
visually located issues, cstyle.sh still shows none.