Backups to SMB no longer crawl; add occ backup:cancel

Each 30-second step opened the location again and read its list of
index files. Over SMB that cost a request per file, so on a large
repository opening took longer than the step: each step stored one
4 MiB pack and the list grew further. A first backup of nc01 to a
Storage Box over SMB managed 12 GiB in 35 hours.

occ backup:run and system cron now keep the repository open for the
whole run (reopened when another process took a step or the location
was edited), opening no longer counts towards the step's time, and
SMB folders are listed in one request.

occ backup:cancel stops a running backup: no new step starts and the
step in progress finishes first. A run of an earlier version, which
does not hold back its next step, is marked cancelled after two
minutes unless it is recording its snapshot.

docs/HOWTO.md

@@ -50,6 +50,14 @@ occ backup:target:list
 occ backup:status
 ```
 
+A backup that hangs or takes far too long can be stopped. `backup:status` shows the run number;
+the command waits until the step in progress is finished, then the process working on the run
+stops, and the next cleanup removes what it stored:
+
+```
+occ backup:cancel 7
+```
+
 Changed password, new host name, or moved the repository? Edit the location instead of adding a
 new one, so its history stays (in the web interface: **Edit** next to the location). The new
 settings must lead to the same repository; options you do not pass keep their value: