feat: add macOS support

[geibos]

Adds macOS support so ipregion.sh runs on Darwin without modification.

What's included

• Bash 4+ guard — macOS ships Bash 3.2. The script now detects an old Bash and re-execs itself with Homebrew's Bash (/opt/homebrew/bin/bash on Apple Silicon, /usr/local/bin/bash on Intel), or prints install instructions and exits if neither is present.
• grep -P replacement — BSD grep has no PCRE support. grep_wrapper --perl now shells out to perl instead, which is present on every macOS install.
• Distro/package manager detection — Darwin maps to macos → brew install.
• Interface detection — there is no ip command; check_ip_interfaces parses ifconfig output on Darwin, skipping loopback (127.) and link-local (fe80:, ::1) addresses.
• head -n -1 / tail -n +3 replacement — BSD head does not accept negative line counts. Replaced with Bash parameter expansion, which behaves identically on both platforms.
• README — added a macOS install section, and corrected the dependency list to name column directly (the util-linux/bsdmainutils package names are Linux-specific; the actual dependency is the column binary).

Testing

Verified on macOS 15 (Darwin 24.6.0, Apple Silicon) with Bash 5.3:

• bash -n ipregion.sh — clean
• ./ipregion.sh --help
• ./ipregion.sh -g primary -4 — full run, all GeoIP services resolved correctly

No Linux behaviour is changed: every new branch is gated on $OS_NAME == "Darwin", except the --perl wrapper and the head/tail replacements, which are portable and produce identical output on both platforms.