Abstract netsnmp_table_dataset_remove_and_delete_row from actual library as well

Another function that used to be in libnetsnmphelpers in net-snmp <5.6 and now is part of
libnetsnmpagent.
Add prototype for netsnmp_table_dataset_remove_and_delete_row
Add missing prototypes for init_mib() and netsnmp_table_dataset_remove_and_delete_row()