sss_iface: prevent from using invalid names that start with digits
https://dbus.freedesktop.org/doc/dbus-specification.html#message-protocol-names
- Bus names that start with a colon (':') character are unique connection names. Other bus names are called well-known bus names.
- Bus names are composed of 1 or more elements separated by a period ('.') character. All elements must contain at least one character.
- Each element must only contain the ASCII characters "[A-Z][a-z][0-9]_-", with "-" discouraged in new bus names. Only elements that are part of a unique connection name may begin with a digit, elements in other bus names must not begin with a digit.
- Bus names must contain at least one '.' (period) character (and thus at least two elements).
- Bus names must not begin with a '.' (period) character.
- Bus names must not exceed the maximum name length (255).
Resolves:
https://pagure.io/SSSD/sssd/issue/3872
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>