Locations: edit connection settings without losing the history

A changed password or host name could only be handled by adding a new
location, which starts an empty repository and, with a retention lock at
the provider, keeps the old one billed. Edit now changes a location in
place (web interface and occ backup:target:edit). The settings are only
saved when they lead to the same repository; a failed connection and a
missing repository give separate errors, and the folder is not created
while checking. Stored passwords are never sent to the browser: an empty
password field keeps the stored value.

docs/HOWTO.md

@@ -50,6 +50,14 @@ occ backup:target:list
 occ backup:status
 ```
 
+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:
+
+```
+occ backup:target:edit nas -o password=<new pw>
+```
+
 ## 3. Run the first backup
 
 ```