Fix the problems reported in SF bug #2668866.

Fix the problems reported in SF bug #2668866.

src/mod-memoserv.c (KEY_ACCOUNTS, KEY_MESSAGES, KEY_FLAGS): Define new
    database key names.
  (MSMSG_CANNOT_SEND_SELF): Define new translation string.
  (MSMSG_NEW_MESSAGE): Add format field for the message ID.
  (memoserv_can_send): Do not allow sending to oneself.  Require that the
    common channel be one that the target has joined at some point.
  (cmd_send): Use the new field in MSMSG_NEW_MESSAGE.
  (cmd_read): Initialize the time_t that is shown.
  (memoserv_saxdb_read_messages): Renamed from memoserv_saxdb_read().
  (memoserv_saxdb_read_accounts): New function.
  (memoserv_saxdb_read): Check for the "accounts" subkey.  If it exists,
    read the account data from that key.
  (memoserv_saxdb_write): Write account flags in a new section.  Use hex
    rather than base64 for the memo IDs.