Support definition of additional sector types in sector.config,

Support definition of additional sector types in sector.config,
missing bits:
(budg, calc_all, update_main): Use SCT_TYPE_MAX instead of SCT_MAXDEF
to size tables.
(map_char): Update sanity check; use CANT_HAPPEN().
(show_sect_build, show_sect_stats, show_sect_capab): Use the sentinel
instead of SCT_MAXDEF.