fix: use one second default ping interval on OpenBSD

[xfocus3]

Summary

• Use a 1.0s default ping interval on OpenBSD, where unprivileged users cannot use sub-second ping intervals.
• Keep the existing 0.2s default on other platforms and the 0.5s --cmd default unchanged.
• Update the --watch-interval help text to mention the OpenBSD-specific default.

Closes #392.

Verification

• cargo fmt --all -- --check
• cargo check -p gping
• cargo test -p gping --no-run