Fixes from the phase-1 code review

- one locking protocol per location: backup steps and restores hold a
  shared lock, prune and untrash an exclusive one and refuse while any
  run (backup, restore, export) exists; starting a backup checks and
  creates the run under the exclusive lock, closing the race with prune
- snapshot entries are validated when read (relative path without '.',
  '..', NUL, backslash or control characters); raw restore also checks
  that the resolved directory stays below the target; the restore API
  and service accept canonical paths only
- recovery kit confirmation requires a recent password confirmation and
  the confirmation code printed in the current kit file (new code per
  kit version), proving the kit was actually downloaded
- prune decides on the authenticated index files on the location and
  refreshes the local index cache from them
- restore job marks the run failed on any error; restore runs without
  progress for 6 hours are failed so they cannot block cleanup forever
- tests for safe paths and a malicious snapshot path

lib/Command/KeyConfirm.php

@@ -40,6 +40,11 @@ class KeyConfirm extends Command {
 		$output->writeln('');
 		$output->writeln(wordwrap(self::STATEMENT, 78));
 		$output->writeln('');
+		$code = (string)$this->getHelper('question')->ask($input, $output, new Question('Confirmation code from the recovery kit file ("confirmation_code"): '));
+		if (!$this->keys->checkKitCode($code)) {
+			$output->writeln('<error>That code does not match the current recovery kit (download it with occ backup:key:kit first).</error>');
+			return 1;
+		}
 		$answer = (string)$this->getHelper('question')->ask($input, $output, new Question('Type "' . self::CONFIRMATION . '" to proceed: '));
 		if (trim($answer) !== self::CONFIRMATION) {
 			$output->writeln('<error>Not confirmed.</error>');