winbindd: only call winbindd_validate_cache when not in offline logon mode.

winbindd: only call winbindd_validate_cache when not in offline logon mode.

originally, the cache was cleared before calling validate, but
this way, we skipt the validation of the database when not in
offline logon mode.

This is put into a new wrapper function winbindd_cache_validate_and_initialize()
which is now called in winbindd.c instead calling validate and
initialize functions separately.

Michael