libc: Add an implementation of strtol/strtoul and export it via plugins

These were lifted from the lua plugin.

sdl, doom, puzzles updated to use the exported version

todo: lua, maybe?
also: convert uses of atoi [back] to strtol

Change-Id: I5a1ebbe8d8c99349e594ab9bbbce474e7645b4e9

apps/plugins/doom/d_deh.c

@@ -2882,7 +2882,7 @@ boolean deh_GetData(char *s, char *k, uint_64_t *l, char **strval, int fpout)
          okrc = FALSE;
       }
       // we've incremented t
-      val = atoi(t);//strtol(t,NULL,0);  // killough 8/9/98: allow hex or octal input
+      val = strtol(t,NULL,0);  // killough 8/9/98: allow hex or octal input
    }
 
    // go put the results in the passed pointers