Add opt-in shell lint and format targets

Introduce make lint-shell / fmt-shell with shellcheck and
shfmt (-i 2 -ci -bn), normalize first-party scripts, and
document the new targets. Keep them out of make lint / fmt.

Signed-off-by: Ludovic Vielle <ludovic@probo.com>

cmd/probo-agent/installer/macos/reinstall.sh

@@ -11,13 +11,13 @@ SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 PKG="${1:-}"
 
 if [ "$(id -u)" -ne 0 ]; then
-    echo "error: must run as root (try: sudo make -C cmd/probo-agent install)" >&2
-    exit 1
+  echo "error: must run as root (try: sudo make -C cmd/probo-agent install)" >&2
+  exit 1
 fi
 
 if [ -z "${PKG}" ] || [ ! -f "${PKG}" ]; then
-    echo "error: usage: $0 /path/to/probo-agent_*.pkg" >&2
-    exit 2
+  echo "error: usage: $0 /path/to/probo-agent_*.pkg" >&2
+  exit 2
 fi
 
 "${SCRIPT_DIR}/uninstall.sh"