homework: allow specifying explicit additional mount options when using CIFS backend

This is useful since certain shares can only be mounted with additional
mount flags. For example the SMB share in modern AVM Fritz!Boxes
requires "noserverino" to be set to work from Linux.

docs/USER_RECORD.md

@@ -415,6 +415,9 @@ mount as home directory of the user on login. Should be in format
 `//<host>/<service>/<directory/…>`. The directory part is optional. If missing
 the top-level directory of the CIFS share is used.
 
+`cifsExtraMountOptions` → A string with additional mount options to pass to
+`mount.cifs` when mounting the home directory CIFS share.
+
 `imagePath` → A string with an absolute file system path to the file, directory
 or block device to use for storage backing the home directory. If the `luks`
 storage is used this refers to the loopback file or block device node to store
@@ -707,15 +710,16 @@ that may be used in this section are identical to the equally named ones in the
 `notAfterUSec`, `storage`, `diskSize`, `diskSizeRelative`, `skeletonDirectory`,
 `accessMode`, `tasksMax`, `memoryHigh`, `memoryMax`, `cpuWeight`, `ioWeight`,
 `mountNoDevices`, `mountNoSuid`, `mountNoExecute`, `cifsDomain`,
-`cifsUserName`, `cifsService`, `imagePath`, `uid`, `gid`, `memberOf`,
-`fileSystemType`, `partitionUuid`, `luksUuid`, `fileSystemUuid`, `luksDiscard`,
-`luksOfflineDiscard`, `luksCipher`, `luksCipherMode`, `luksVolumeKeySize`,
-`luksPbkdfHashAlgorithm`, `luksPbkdfType`, `luksPbkdfTimeCostUSec`,
-`luksPbkdfMemoryCost`, `luksPbkdfParallelThreads`, `rateLimitIntervalUSec`,
-`rateLimitBurst`, `enforcePasswordPolicy`, `autoLogin`, `stopDelayUSec`,
-`killProcesses`, `passwordChangeMinUSec`, `passwordChangeMaxUSec`,
-`passwordChangeWarnUSec`, `passwordChangeInactiveUSec`, `passwordChangeNow`,
-`pkcs11TokenUri`, `fido2HmacCredential`.
+`cifsUserName`, `cifsService`, `cifsExtraMountOptions`, `imagePath`, `uid`,
+`gid`, `memberOf`, `fileSystemType`, `partitionUuid`, `luksUuid`,
+`fileSystemUuid`, `luksDiscard`, `luksOfflineDiscard`, `luksCipher`,
+`luksCipherMode`, `luksVolumeKeySize`, `luksPbkdfHashAlgorithm`,
+`luksPbkdfType`, `luksPbkdfTimeCostUSec`, `luksPbkdfMemoryCost`,
+`luksPbkdfParallelThreads`, `rateLimitIntervalUSec`, `rateLimitBurst`,
+`enforcePasswordPolicy`, `autoLogin`, `stopDelayUSec`, `killProcesses`,
+`passwordChangeMinUSec`, `passwordChangeMaxUSec`, `passwordChangeWarnUSec`,
+`passwordChangeInactiveUSec`, `passwordChangeNow`, `pkcs11TokenUri`,
+`fido2HmacCredential`.
 
 ## Fields in the `binding` section