Add locking mechanism

Action won't start if one is already running.  This should prevent
overlapping actions.

It's possible to turn off locking to allow running imapdomo as service,
ie. locking will be taken care of by init system.

src/imapdomo.skel

             "-c DIR      change to DIR before doing anything"                 \
             "-l          list handlers and exit"                              \
             "-d          turn on debugging mode"                              \
+            "-N          turn off locking"                                    \
        --                                                                     \
        "imapdomo will try to read init.lua and handlers/ACTION.lua from its"  \
        "configuration directory."                                             \
       | sed 's/.lua$//'
 }
 
+lock() {
+    #
+    # Put the lockfile
+    #
+    $NoLock && return 0
+    date +"$$@%s" > "$IMAPDOMO_USER_CACHE/lock"
+}
+
+is_locked() {
+    #
+    # Does lockfile exist?
+    #
+    # True if lockfile exists. False if lockfile does not exist or locking
+    # is turned off.
+    #
+    $NoLock && return 1
+    test -e "$IMAPDOMO_USER_CACHE/lock"
+}
+
+unlock() {
+    #
+    # Remove the lockfile
+    #
+    $NoLock && return 0
+    rm "$IMAPDOMO_USER_CACHE/lock"
+}
+
 handle() {
     #
     # Handle action $Action
     local CdTo      # change dir to this before running imapfilter
     local Certs     # certificate storage
     local es=0      # exit status of this function
+    local NoLock    # disable locks
     CfgDir="$IMAPDOMO_CFGDIR"
     LogDir="$IMAPDOMO_USER_CACHE/logs"
     HeaderDir="$IMAPDOMO_USER_CACHE/headers"
     Certs="$IMAPDOMO_CFGDIR/certificates"
+    NoLock=false
     Debug=false
     #shellcheck disable=SC2034
     while true; do case $1 in
         -d) Debug=true; PRETTY_DEBUG=true; shift ;;
         -t) Certs="$2"; shift 2 || usage -w "missing value to: $1" ;;
         -l) lshandlers; exit ;;
+        -N) NoLock=true; shift ;;
         -V|--version-semver) show_semversion ;;
         --version) show_version ;;
         -*) usage -w "unknown argument: '$1'" ;;
     esac done
     Action="$1"; shift
     test -n "$Action" || usage -w "no action specified"
-    debug -v Action CfgDir LogDir HeaderDir Debug CdTo
+    debug -v Action CfgDir LogDir HeaderDir Debug CdTo NoLock
+    is_locked && return 1
+    lock
     handle "$Action"; es=$?
+    unlock
     return $es
 }