curses: include hostname in title

[Komzpa]

Summary

• include the local hostname in the centered curses title
• keep the existing source/destination line below unchanged

This implements the small title change requested in #121 so multiple running mtr instances are easier to distinguish.

Fixes #121.

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 .
• script -q -e -c "sh -c 'sleep 1; printf q' | ./mtr --curses --no-dns --max-ttl 1 localhost" /tmp/mtr-title.typescript and verified the title contains My traceroute on $(hostname)

[Komzpa]

Superseded by #576, which now carries the curses title context together with the related curses display polish.