Add missing option description in usage message

src/imapdomo.skel

             "-c DIR      change to DIR before doing anything"                 \
             "-l          list handlers and exit"                              \
             "-d          turn on debugging mode"                              \
+            "-t CERTS    Use certificates file CERTS."                        \
             "-N          turn off locking; make sure that handlers cannot"    \
             "            interfere with each other or that another locking"   \
             "            mechanism is in place.  Also see NOTE below."        \
+            "-V          Show imapdomo version and exit."                     \
        --                                                                     \
        "imapdomo will try to read init.lua and handlers/ACTION.lua from its"  \
        "configuration directory."                                             \