Add a new irc_user_modes() helper function and irc_user_mode_chars[] array.

Add a new irc_user_modes() helper function and irc_user_mode_chars[] array.

src/proto.h (irc_user_modes): Declare.
  (irc_user_mode_chars): Likewise.
src/proto-common.c (irc_user_modes): Define.
src/proto-bahamut.c (irc_user_mode_chars): Define.
  (irc_user): Use irc_user_mode_chars() instead of open-coding it.
src/proto-p10.c (irc_user_mode_chars): Define.
  (irc_user): Use irc_user_mode_chars() instead of open-coding it.
src/opserv.c (cmd_whois): Use irc_user_mode_chars() here too.
src/modcmd.c (modcmd_saxdb_write): Write bot modes to the database.
  (modcmd_load_bots): Read them out and use them.