db.describe: add JSON support

[kritibirda26]

Add JSON support to db.describe module. The JSON output looks like:

{
    "table": "mysoils",
    "description": "",
    "insert": null,
    "delete": null,
    "ncols": 7,
    "nrows": 1428,
    "columns": [
        {
            "position": 1,
            "column": "cat",
            "description": "",
            "type": "INTEGER",
            "length": 20,
            "scale": 0,
            "precision": 0,
            "default": null,
            "nullok": true,
            "select": null,
            "update": null
        },
        {
            "position": 2,
            "column": "OBJECTID",
            "description": "",
            "type": "INTEGER",
            "length": 20,
            "scale": 0,
            "precision": 0,
            "default": null,
            "nullok": true,
            "select": null,
            "update": null
        },
        {
            "position": 3,
            "column": "AREA",
            "description": "",
            "type": "DOUBLE PRECISION",
            "length": 20,
            "scale": 0,
            "precision": 0,
            "default": null,
            "nullok": true,
            "select": null,
            "update": null
        },
        {
            "position": 4,
            "column": "PERIMETER",
            "description": "",
            "type": "DOUBLE PRECISION",
            "length": 20,
            "scale": 0,
            "precision": 0,
            "default": null,
            "nullok": true,
            "select": null,
            "update": null
        },
        {
            "position": 5,
            "column": "GSLNC250_",
            "description": "",
            "type": "INTEGER",
            "length": 20,
            "scale": 0,
            "precision": 0,
            "default": null,
            "nullok": true,
            "select": null,
            "update": null
        },
        {
            "position": 6,
            "column": "GSLNC250_I",
            "description": "",
            "type": "INTEGER",
            "length": 20,
            "scale": 0,
            "precision": 0,
            "default": null,
            "nullok": true,
            "select": null,
            "update": null
        },
        {
            "position": 7,
            "column": "GSL_NAME",
            "description": "",
            "type": "CHARACTER",
            "length": 6,
            "scale": 0,
            "precision": 0,
            "default": null,
            "nullok": true,
            "select": null,
            "update": null
        }
    ]
}

Where ??? was used as special string for unknown in plain format, I have changed it to null. For instance, select, update, delete and insert privilege information field.

[kritibirda26]

@cwhite911 the pr is ready for review.

[cwhite911]

Looks good.

[kritibirda26]

@echoix can you please merge this PR?

[petrasovaa]

Looks like it triggered this:

New defect(s) Reported-by: Coverity Scan
Showing 1 of 1 defect(s)


** CID 1559529:  Memory - illegal accesses  (UNINIT)
/db/db.describe/main.c: 86 in main()


________________________________________________________________________________________________________
*** CID 1559529:  Memory - illegal accesses  (UNINIT)
/db/db.describe/main.c: 86 in main()
80     
81         if (db_describe_table(driver, &table_name, &table) != DB_OK)
82             G_fatal_error(_("Unable to describe table <%s>"),
83                           db_get_string(&table_name));
84     
85         if (!parms.printcolnames)
>>>     CID 1559529:  Memory - illegal accesses  (UNINIT)
>>>     Using uninitialized value "root_object" when calling "print_table_definition".
86             print_table_definition(driver, table, parms.format, root_object,
87                                    cols_array);
88         else {
89             ncols = db_get_table_number_of_columns(table);
90     
91             db_init_string(&stmt);