gh-61: Fix context scripts locking

Use directory lock instead of file lock. Ensure the environment is
cleaned (e.g. the CONTEXT ISO is unmounted) and lock is cleaned
via bash trap in case of failure.

Closes #61.

base/usr/sbin/one-contextd

@@ -92,25 +92,29 @@ function end_context {
     [ -e "${CONTEXT_NEW}" ] && rm "${CONTEXT_NEW}"
 }
 
-function adquire_lock {
-    while [ -e "${LOCK_FILE}" ]; do
+function acquire_lock {
+    while true; do
+        if mkdir "${LOCK_FILE}" 2>/dev/null; then
+            trap 'cleanup' EXIT
+            break
+        fi
+
         sleep 1
     done
-
-    touch "${LOCK_FILE}"
 }
 
 function release_lock {
-    rm -f "${LOCK_FILE}"
+    rm -rf "${LOCK_FILE}"
+}
+
+function cleanup {
+    end_context
+    release_lock
 }
 
 COMMAND="$1"
 
-adquire_lock
+acquire_lock
 
 get_new_context
 check_context && run_context "$COMMAND"
-end_context
-
-release_lock
-