r24879: Activate the winbindd cache-validation message handler.

r24879: Activate the winbindd cache-validation message handler.
Now the winbindd cache can be checked at runtime by
calling "smbcontrol winbindd validate-cache".

For the execution of the validation code, I fork a child
and in the child restore the default SIGCHLD handler in
order for the fork/waitpid mechanism of tdb_validate to work.

Michael