some coordxy and other conversion warnings

When dist2() got changed to use coordxy parameters, a macro that uses
it in its definition was overlooked and it had (int) casts in it.
That caused a warning about possible data loss when the int
then got converted to coordxy for the dist2() call.

Give online2() coordxy parameters instead of int, like its bretheren.

Avoid a couple of implicit conversion warnings where ints were being assigned
to smaller uchar or ints being assigned to smaller short.

A couple of signed vs unsigned warnings on some rumor processing.

Avoid some signed vs unsigned warnings in mdlib/makedefs where a signed int
param eventually got used in an external call that took size_t.
Eliminate all of it by just having the outer NetHack routine also take
a size_t.

Lastly, insert some default C99 alternative time-related code
in mdlib/makedefs since asctime() and ctime() are being flagged as
deprecated in the upcoming C23 standard and will now start to trigger
warnings for anyone using a C23-compliant compiler.

src/hacklib.c

@@ -721,7 +721,7 @@ isqrt(int val)
 
 /* are two points lined up (on a straight line)? */
 boolean
-online2(int x0, int y0, int x1, int y1)
+online2(coordxy x0, coordxy y0, coordxy x1, coordxy y1)
 {
     int dx = x0 - x1, dy = y0 - y1;
     /*  If either delta is zero then they're on an orthogonal line,