Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Initializate "offset" variable to zero; otherwise, if not using
the -o option, it might contain garbage.

MFC after: 1 day

o Add an option allowing to start search from some offset instead of offset 0;

o mesure distance between UFS1 and UFS2 superblocks independently.

MFC after: 1 week

o Make it possible to compile find-sb on non-FreeBSD system (MacOS X in my
case). Can be useful for recovering in some cases;

o use SBLOCK_UFS2 instead of SBLOCK_UFS1 to calculate likely start of the UFS2
partition from the beginning of disk.

Yesterday, FreeBSD-current ate my disklabel.
Today, I wrote a program to tell me where the partitions used to be.
Now I have my /media filesystem back.