scan: never record its own database files (closes #115)

When the database lay under a scanned operand, the walk recorded the
database file, its -wal and -shm files and its .lock file like any other
file, so every rescan updated them and report paired them with empty
files. Once the database and the lock file are open, the scan now notes
their device and inode, and the walk passes over a file with one of
them, as an operand or below one, printing no line and counting
nothing. Matching by device and inode holds through a symlinked
directory, where comparing paths would not. A record an earlier scan
left for one of them is deleted as unverified. README's smoke-test note
on keeping the database out of the tree is gone, as it no longer holds.

Judgement call: walkRules, in exclude.go, carries the noted files to the walk.

Model: opus-5-5

exclude.go

@@ -43,14 +43,17 @@ const typesPerHelpLine = 6
 // readMountInfo reads.
 var errMountInfoLine = errors.New("unexpected line")
 
-// walkRules holds the two rules for the walk that a scan sets up as it
+// walkRules holds the rules for the walk that a scan sets up as it
 // starts. excludes are the default excludes, or none with
 // --no-default-excludes. nonPersistent maps the device ID of every
 // mounted filesystem that is not persistent to its type; the scan
 // records and reads nothing on one of those, whatever the flags.
+// databaseFiles are the scan's own database files, which the walk
+// passes over (see databaseFiles in db.go).
 type walkRules struct {
 	excludes      []string
 	nonPersistent map[uint64]string
+	databaseFiles []fileRec
 }
 
 // defaultExcludes returns the default excludes of the system goos