cmd/syncthing: Implement generate as a subcommand with optional API credential setting (fixes #8021) (#8043)

Accept a subcommand as an alternative to the --generate option.  It
accepts a custom config directory through either the --home or
--config options, using the default location if neither is given.

Add the options --gui-user and --gui-password to "generate", but not
the "serve --generate" option form.  If either is given, an existing
config will not abort the command, but rather load, modify and save it
with the new credentials.  The password can be read from standard
input by passing only a single dash as argument.

Config modification is skipped if the value matches what's already in
the config.

* cmd/syncthing: Utilize lib/locations package in generate().
Instead of manually joining paths with "magic" file names, get them
from the centralized locations helper lib.

* cmd/syncthing: Simplify logging for --generate option.
Visible change: No more timestamp prefixes.

lib/syncthing/utils.go

@@ -24,6 +24,29 @@ import (
 	"github.com/syncthing/syncthing/lib/tlsutil"
 )
 
+func EnsureDir(dir string, mode fs.FileMode) error {
+	fs := fs.NewFilesystem(fs.FilesystemTypeBasic, dir)
+	err := fs.MkdirAll(".", mode)
+	if err != nil {
+		return err
+	}
+
+	if fi, err := fs.Stat("."); err == nil {
+		// Apprently the stat may fail even though the mkdirall passed. If it
+		// does, we'll just assume things are in order and let other things
+		// fail (like loading or creating the config...).
+		currentMode := fi.Mode() & 0777
+		if currentMode != mode {
+			err := fs.Chmod(".", mode)
+			// This can fail on crappy filesystems, nothing we can do about it.
+			if err != nil {
+				l.Warnln(err)
+			}
+		}
+	}
+	return nil
+}
+
 func LoadOrGenerateCertificate(certFile, keyFile string) (tls.Certificate, error) {
 	cert, err := tls.LoadX509KeyPair(certFile, keyFile)
 	if err != nil {