Name the database each command uses on stderr (closes #111)

scan, report and trees now print one line on stderr before anything
else, naming the database and whether its path is the default or comes
from SFDUPES_DATABASE; scan also says when the file is new. report and
trees print it before opening the database, so a missing-database error
follows it. A relative path gets the working directory put in front of
it without cleaning: cleaning drops a ".." with the directory before
it, which names another file when that directory is a symlink. README
"Database" lists when scan writes each file.

Unverified: the line on a terminal has no test of its own.
Rule suppressed: TestDatabasePath carries nolint:paralleltest, as it
changes the working directory.

Model: opus-5-5

db_test.go

@@ -42,6 +42,7 @@ func openTestDB(t *testing.T) *sql.DB {
 	return db
 }
 
+//nolint:paralleltest // changes the process-wide working directory
 func TestDatabasePath(t *testing.T) {
 	// README §Database: SFDUPES_DATABASE when set and non-empty, the
 	// default otherwise, with the working directory put in front of a