libsmartcols: introduce default sort column

libsmartcols: introduce default sort column

* add default sort column, set by scols_sort_table()

* sort tree according to default sort column also in scols_sort_table_by_tree()

The function scols_sort_table() does not sort tree branches if tree
is not enabled. The function scols_sort_table_by_tree() does not care
if tree is enabled and it always follows parent->child relations. For
scols_sort_table_by_tree() we need to follow order in branches if
previously scols_sort_table() has been called.

For example lsblk calls

	scols_sort_table(tb, cl);
	scols_sort_table_by_tree(tb);

for list-like output (for example lsblk -P) and users assume the
same order as for tree (lsblk --tree).

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1940607
Signed-off-by: Karel Zak <kzak@redhat.com>