For any btrfs-$FOO executable, rename the main source file from
$FOO.c to to btrfs-$FOO.c
This makes it slightly more obvious what's building what,
and allows us to write a default rule in the Makefile for
these tools.
(also add btrfs-calc-size to the list of objects to remove
on make clean)
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
