btrfs-show-super: don't try to print not-superblocks

btrfs-show-super: don't try to print not-superblocks

If we point btrfs-show-super at a not-btrfs-device and
try to print all superblocks, bad things are apt to happen:

superblock: bytenr=274877906944, device=/dev/sdc2
---------------------------------------------------------
btrfs-show-super: ctree.h:1984: btrfs_super_csum_size: \
Assertion `!(t >= (sizeof(btrfs_csum_sizes) / sizeof((btrfs_csum_sizes)[0])))' failed.
csum			0xAborted

Don't try to print superblocks that don't look like superblocks,
and add an "-f" (force) option to try anyway, if the user
really wants to give it a shot.

Fix some spelling & capitalization while we're at it.

The manpage says that if any problem happens, 1 will
be returned, but that's already not true today LOL, so
I didn't bother to make it true when we detect bad
sb magic, either...

I figure it's worth continuing and trying all superblocks
in case just one has a corrupt magic.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
[renamed -f to -F due to clash with existing option, converted
relevant docs to asciidoc]
Signed-off-by: David Sterba <dsterba@suse.cz>