shorewall6-lite: fix build on macos

shorewall6-lite: fix build on macos

shorewall6-lite macos build fails due to:
1. MacOS bash is too old (3.x), but shorewall6-lite 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

3. fakeuname does not work in install.sh because install.sh
redefines PATH.
This patch removes PATH=... from install.sh on macos

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