btrfs-progs: Add helpers functions to handle the printing of data in tabular format

btrfs-progs: Add helpers functions to handle the printing of data in tabular format

This patch adds some functions to manage the printing of the data in
tabular format.

The function
	struct string_table *table_create(int columns, int rows)
creates an (empty) table.

The functions
	char *table_printf(struct string_table *tab, int column,
		int row, char *fmt, ...)
	char *table_vprintf(struct string_table *tab, int column,
		int row, char *fmt, va_list ap)
populate the table with text. To align the text to the left, the text
shall be prefixed with '<', otherwise the text shall be prefixed by a
'>'. If the first character is a '=', the the text is replace by a
sequence of '=' to fill the column width.

The function
	void table_free(struct string_table *)
frees all the data associated to the table.

The function
	void table_dump(struct string_table *tab)
prints the table on stdout.

Signed-off-by: Goffredo Baroncelli <kreijack@inwind.it>
Signed-off-by: David Sterba <dsterba@suse.cz>