Add account notes.

Add account notes.

src/nickserv.h (struct handle_note): New structure.
  (struct handle_info): Add field to point to account's notes.

src/nickserv.c (KEY_NOTE*): New configuration key macros.
  (WALK_NOTES): New macro to make a single pass through notes while
    deleting ones that have expired.
  (msgtab): Add entries to support commands related to account notes.
  (free_handle_info): Release notes on an account.
  (cmd_handleinfo): Show notes, or say there are none.
  (cmd_addnote): New command-handler function.
  (cmd_delnote): New command-handler function.
  (nickserv_saxdb_write): Write notes to the database.
  (nickserv_db_read_handle): Read notes from the database.
  (init_nickserv): Register new functions.

src/nickserv.help (ADDNOTE): Document new command.
  (DELNOTE): Likewise.