MFC: Sync restore with the version in -current.

Fix remaining nits from PR bin/39905.
Check for write failure while building temp files.
Correctly set file group when restore is run by a user other than root.
Use safer string handling.
Fixes the "getfile: lost data" panic when restoring older dumps.
Fix spurious "Header with wrong dumpdate" message.
Add -D for degraded mode, which tries harder to read corrupted backups.
Remove a diagnostic message from man page that is no longer printed.