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.
double-remainder problem: x-coordinate SHRT_MAX+1 is truncated to 0 by
cast to coord, then converted by xabs(). This is wrong unless WORLD_X
divides SHRT_MAX+1.
(sarg_xy, sarg_getrange, sarg_range): Use them.
(inputxy): No longer used, remove.
(sarg_type): Use NS_UNDEF instead of 0.
(sarg_list): Change confusing loop control. Properly diagnose
overlong lists; used to silently ignore list tail and return MAX+1,
which made a later snxtitem_list() fail.
(atoip): No longer used, remove. Parsing was broken anyway.
(sarg_type, sarg_xy, sarg_area, sarg_range, sarg_list, sarg_getrange):
Use plain char * instead of s_char *.
