more: align MORE_SHELL_LINES semantics with less(1)

more: align MORE_SHELL_LINES semantics with less(1)

Change MORE_SHELL_LINES to subtract lines from lines_per_screen
(like LESS_SHELL_LINES in less(1)) rather than setting the absolute
number of lines per screenful.

The block is moved after lines_per_screen is finalized (either from
the default lines_per_page - 1 or from an explicit -n/--lines), so
it correctly adjusts both cases. Values that are zero or would exceed
lines_per_screen are silently ignored.

Addresses: https://github.com/util-linux/util-linux/issues/3476
Co-authored-by: Christian Goeschel Ndjomouo <cgoesc2@wgu.edu>
Signed-off-by: Karel Zak <kzak@redhat.com>