sql.py: pkey_col(): Documented that if there is no pkey, returns the first column in the table

sql.py: pkey_col(): Specify recover directly as a kw_arg because it's the only kw_arg passed to pkey_name()

sql.py: Added table_pkey_col() and use it in pkey_name()

sql.py: Renamed pkey() to pkey_name()

sql.py: Renamed pkey_col_() to pkey_col()

sql.py: Removed no longer used pkey_col

db_xml.py: cleanup_table(): Inline sql.pkey_col ('row_num') because this is the only place it's used

cleanup_table(): Use new sql.table_cols() instead of sql.table_col_names()

sql.py: Added table_cols()

db_xml.py: put(): Fixed bug where needed to avoid truncating the pkeys_loc table, in case it's the same as one of the in_tables. This occurs now that sql_io.put_table() passes through the actual input column instead of the joined-together input table's column when ignoring all rows.