Make alternative of FileUtils

Updated: StringUtil.match_regexp is now easy to replace with `Fluent::Plugin::Base#string_safe_encoding`.
There are still `Fluent::FileUtil.writable?` and `#writable_p?`.

`Fluent::Plugin::StringUtil.match_regexp` (implementation was moved to `Fluent::Compat::StringUtil` now) has some implementation problem:
- it provides encode-safety about regular expression matching, but method name doesn't show it
- it uses `$log`, but should use plugin logger
- scrubbing strings is very universal process: not only for regular expression matching

My idea is to add a method like this:

``` ruby
def ensure_safe_encoding(str)
begin
yield str
rescue ArgumentError => e
raise e unless e.message.include?("invalid byte sequence in".freeze)
log.info "invalid byte sequence is replaced in `#{string}`"
str = str.scrub('?')
retry
end
end
```

We can use this method like below:

``` ruby
m = ensure_safe_encoding(line){|s| s =~ /my pattern/}
# or other methods or ...
chars = ensure_safe_encoding(line){|s| s.split }
```

Blockers for implementation are:
- Naming problem of the method
- Namespace? `lib/fluent`? `lib/fluent/util`? or...
- Should it be a method of `Fluent::Plugin::Base`? plugin helper? (If true, we'll have so many helpers...) or just Mixin? (We're eliminating mixins, but...)