*: use ul_default_shell() for interactive shell spawning

*: use ul_default_shell() for interactive shell spawning

Update tools that spawn interactive shells to use ul_default_shell()
for consistent shell resolution. This ensures these tools respect both
$SHELL and the user's configured shell from the passwd database before
falling back to _PATH_BSHELL.

Affected tools:
- script(1): fixes history truncation when invoked without $SHELL
- scriptlive(1): consistent with script(1) behavior
- flock(1): for -c command execution
- more(1): for shell escape feature
- exec_shell (used by unshare(1) and nsenter(1))

This change addresses user reports of data loss due to tools defaulting
to /bin/sh instead of the user's configured shell, particularly affecting
command history with different HISTSIZE configurations.

Addresses: https://github.com/util-linux/util-linux/issues/3865
Signed-off-by: Alessandro Ratti <alessandro@0x65c.net>