curses: add report snapshot on exit

[Komzpa]

Summary

• add an opt-in --report-on-exit flag for the curses interface
• close curses normally, then print a standard report snapshot so the last trace remains visible in terminals that use an alternate screen
• document the new option in mtr(8) and --help

This follows the workaround discussed in #323 without changing the default curses behavior for existing users.

Fixes #323.

Validation

• git diff --check
• ./bootstrap.sh
• ./configure --without-gtk --without-jansson
• make -j "$(nproc)"
• sudo python3 ./test/cmdparse.py
• uv run --python 3.9 --with flake8==3.9.2 --with flake8-bandit==2.1.2 --with bandit==1.7.2 --with pbr==7.0.3 python -m flake8 .
• ./mtr --help 2>&1 | grep -F -- --report-on-exit
• script -q -e -c "sh -c 'sleep 1; printf q' | ./mtr --report-on-exit --curses --no-dns --max-ttl 1 localhost" /tmp/mtr-report-on-exit.typescript

[Komzpa]

Superseded by #580, which now carries both curses startup/exit controls so the merge stack no longer has an internal option-plumbing conflict.