If you want to declare a pointer which the address pointed to is constant,
you should declare it as like `static const char *const var = "...";`.
This commit supplies missing `const` and prevents some programming
error in the future.
On 59818fb, MAX_RADIUS was changed from 15 to 16 to intend to support
"radius 0". But MAX_RADIUS doesn't means the range but outer bound of
the radius table, so it should not be changed, and this change led
possible out-of-bound access on view_from() and do_clear_area() in
vision.c.
This commit reverts the change and avoids the problem.
SHIRAKATA Kentaro