Add "unreviewed" flag for channels.

Add "unreviewed" flag for channels.

src/chanserv.c (CHANNEL_DEFAULT_FLAGS): Include "unreviewed" flag.
  (CHANNEL_PRESERVED_FLAGS): New macro for flags not reset by !set defaults
  (msgtab): New entry for "CSMSG_SET_UNREVIEWED".
  (chanserv_search_create): Recognize "unreviewed" flag.
  (chan_opt_unreviewed): New function.
  (chan_opt_defaults): Preserve CHANNEL_PRESERVED_FLAGS.
  (cmd_set): Act like cmd_joiner with respect to updating argv.
  (init_chanserv): Register unreviewed option and its subcommands.

src/chanserv.h (CHANNEL_UNREVIEWED): New flag.

src/chanserv.help: Update documentation for SEARCH and SET.