Libravatar: Refactor cache handling to avoid unncessary stat

Libravatar: Refactor cache handling to avoid unncessary stat

The basic pattern of the code we are dealing with is:

  my $path = cache_location(...);
  if (valid_cache($path)) {
    ...;
  }

With cache_location possibly doing a stat() of $path and valid_cache
doing at least one stat() as well.  Rewrite it so that cache_location
always stats and checks the cache (since we always need it).  This way
we can now guarantee this flow requires at *most* 3 stats rather than
5.

The replacement for the above code becomes:

  my ($path, $is_valid) = cache_location(...);
  if ($is_valid) {
     ...;
  }

Signed-off-by: Niels Thykier <niels@thykier.net>