Limit number of opened file globally with --bt-max-open-files option

This change changes the behavior of --bt-max-open-files. Previously,
it specifies the maximum number of opened files for each multi-file
download. Since it is more useful to limit the number globally, the
option now specifies the global limit. This change suggests that
aria2.changeOption() method now ignores --bt-max-open-files and
aria2.changeGlobalOption now reads it and dynamically change the
limit.