core: Config and XML tweaks needed for geolocation

Added:

Replace a variable in a list:
int ast_variable_list_replace_variable(struct ast_variable **head,
    struct ast_variable *old, struct ast_variable *new);
Added test as well.

Create a "name=value" string from a variable list:
'name1="val1",name2="val2"', etc.
struct ast_str *ast_variable_list_join(
    const struct ast_variable *head, const char *item_separator,
    const char *name_value_separator, const char *quote_char,
    struct ast_str **str);
Added test as well.

Allow the name of an XML element to be changed.
void ast_xml_set_name(struct ast_xml_node *node, const char *name);

Change-Id: I330a5f63dc0c218e0d8dfc0745948d2812141ccb