Useless, remove. Users changed to check nchr's sentinel, because
that's clearer than comparing the index to N_MAXNUKE or sizeof(nchr) /
sizeof(*nchr).
(nchr): Change sentinel's n_name to NULL.
NUK_BLD_WORK): New, to encapsulate the formula in one place.
(SCT_MINEFF): New. Use it instead of literal 20.
(buil): Use CANT_HAPPEN() for internal error.
(build_ship, build_land, build_bridge, build_nuke, build_plane,
build_tower): Simplify avail calculation.
(lupgr, supgr, pupgr): Charge 15% of total build avail instead of an
undocumented amount computed from build lcm and hcm.
(build_ship, build_land, build_bridge, build_nuke, build_plane,
build_tower, lupgr, supgr, pupgr): Fix reporting of required avail
when there's not enough. Closes#942823.
(lupgr, supgr, pupgr): Code printed cost rounded down, but required
and charged unrounded cost. Confusing. Round cost down, like the
build command. Closes#942811.
(UPGR_COST, UPGR_EFF, lupgr, supgr, pupgr): New UPGR_COST, UPGR_COST replace
literal values.
(show_bridge, show_tower): Don't claim bridge building requires
`workers' (whatever that is), since it doesn't.
