Players can fire two kinds of tracers. The first is used to check for a
target and e.g. gray out your quivered spell if it has no target.
The second is fired by functions like `player_tracer` and warns
when there are bad targets (e.g. yourself or an ally). Instead of
choosing between these two by setting a combination of flags on a bolt
before firing it, choose between them by passing a different tracer
class to the bolts fire method.

when firing a tracer with the `player_tracer` function don't just
warn about one of your allies that would be hit, instead warn about all
of them in the same way that melee attacks do (e.g. when using the
singing sword). Also, don't spam warnings when firing a starbust that
will target you multiple time, instead just say up to how many times
the starbust can hit you.

Fixes crawl#4151