Pre-beta review: safer destructive commands and hardening

- backup:restore:full refuses on an installation with more than one user
  and asks before it starts (--force skips both, also to rerun after a
  failure halfway), and warns when the snapshot's database dump was not
  one consistent transaction.
- backup:user:restore --mode=replace asks before deleting an existing
  account; non-interactive use needs --force.
- The recovery kit file is created owner-only and never over an existing
  file or symlink; backup:key:import shows the deletion delay it read from
  the kit.
- Adding a location through the web API needs password confirmation, like
  removing one.
- Key derivation parameters from a kit or repository config are bounded,
  and a stalled network stream no longer spins at full CPU.
- Floating-point columns are dumped exactly instead of with 14 digits.

lib/Crypto/StreamCipher.php

@@ -122,6 +122,7 @@ final class StreamCipher {
 	/** fread() may return short reads on network streams; keep reading until $length or EOF. */
 	private static function readFull($in, int $length): string {
 		$buffer = '';
+		$empty = 0;
 		while (strlen($buffer) < $length && !feof($in)) {
 			$chunk = fread($in, $length - strlen($buffer));
 			if ($chunk === false) {
@@ -132,8 +133,15 @@ final class StreamCipher {
 				if (feof($in)) {
 					break;
 				}
+				// A stalled network stream returns '' without EOF forever: back off, then give up
+				// (about a minute) instead of spinning at full CPU until the lease expires.
+				if (++$empty > 600) {
+					throw new CryptoException('Read stalled');
+				}
+				usleep(100000);
 				continue;
 			}
+			$empty = 0;
 			$buffer .= $chunk;
 		}
 		return $buffer;