Fix ldap child memory hierarchy and other issues
    
    The timeout handler was not a child of the request so it could fire even though
    the request was already freed.
    
    The code wouldn't use async writes to the children so it could incur in a short
    write with no way to detect or recover from it.
    Also fixed style of some helper functions to pass explicit paramters instead of
    a general structure.
    Add common code to do async writes to pipes.
    Fixed async write issue for the krb5_child as well.
    
    Fix also sdap_kinit_done(), a return statement was missing and we were mixing
    SDAP_AUTH and errno return codes in state->result
    
    Remove usless helper function that just replicates talloc_strndup()