libsmartcols: add scols_table_calculate(), pager: add less --header support

libsmartcols: add scols_table_calculate(), pager: add less --header support

Add scols_table_calculate() public API to allow column width calculation
without printing. This enables callers to query column widths (via
scols_column_get_width()) before starting output, which is needed to
set up "less --header" for freezing table headers and the first column.

The new function runs __scols_initialize_printing() (which includes
__scols_calculate()) and sets an is_calculated flag so that subsequent
scols_print_table() calls skip redundant recalculation.

Add pager_open_header() to lib/pager.c that sets the LESS environment
variable with "--header N,M" options before spawning the pager process.
This freezes the first N lines (header row) and first M character columns
(first table column) when scrolling in less.

Addresses: https://github.com/util-linux/util-linux/issues/3542