sh: Add internet-check base script

Added script to sh base package to allow checking for internet
connectivity. Checks both through system virtual filesystem and, as a
fallback, through pinging google dns.

Added a simple readme to sh module.

sh/.local/bin/check-internet

@@ -0,0 +1,24 @@
+#!/usr/bin/env sh
+#
+# Check for internet connectivity.
+# Returns 0 if connectivity exists, 1 if it does not.
+
+hasconnec=0
+for intf in /sys/class/net/*; do
+    case $intf in
+    "/sys/class/net/lo") break ;;
+    *)
+        [ "$(cat "$intf/carrier")" = 1 ] && hasconnec=1
+        ;;
+    esac
+done
+
+if [ $hasconnec -ne 1 ]; then
+    ping -q -w 1 -c 1 8.8.8.8 >/dev/null 2>&1 && hasconnec=1
+fi
+
+if [ $hasconnec -eq 1 ]; then
+    exit 0
+else
+    exit 1
+fi