shorewall6 macos build fails due to:
1. MacOS bash is too old (3.x), but shorewall6 requires bash>4
This patch uses OpenWrt tools/bash built for macos (bash 5.x)

2. install.sh detects Darwin using uname and changes install logic,
but it fails in case of cross-platform build
This patch uses fakeuname/host tool to avoid Darwin detection

Signed-off-by: Sergey V. Lobanov <sergey@lobanov.in>