Fix root path handling for WebDAV ext storage

Added missing cleanPath() call that converts "/" to "" when calling
SabreDAV. This is needed because SabreDAV will discard its base URL when
passing "/".
user_ldap: Reimplement convertSID2Str() without BCMath dependency.

Also explicitly format sub-id integers as unsigned, which is required for
32-bit systems.
Merge branch 'stable7-upstream' into stable7-rotdrop