other. Ensure headers in include/ can be included in any order
(except for econfig-spec.h, which is special). New header types.h to
help avoid inclusion cycles. Sort include directives. Remove some
superflous includes.
files ship.config, plane.config, land.config, nuke.config.
(empfile): Declare the new config files.
(install, installdirs): Install them.
(builtins): New make variable.
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.
