updatedb(8): Be simple and search from "/" by default

updatedb(8): Be simple and search from "/" by default

This reverts the behavior change made in commit
6877ea2d2f7caf9b13202f2524abf3b3cf815b45.

That change causes HAMMER PFSes being ignored from indexing, because the
PFSes, which are null-mounted though, behave like different devices and
thus are ignored by "find -x" (the "-x" option prevents find entering
directories with a different device number).

Workaround or properly deal with the PFSes would be too complex, which I
don't think it's a good idea,  therefore revert to the original behavior.

If one person really wants to ignore the root filesystem but have other
directories indexed, he/she should know to configure the search paths.

Meanwhile, tweak comments and fix whitespaces.