configure: Add options for bash-completion install

As was shown in a41d4e29 (comment 65923) installation of bash-completion script may result in error if user doesn't have permisions to write in this dir. So I suggest to make user explicitly opt-in to install bash-completion script with --enable-bashcompletion option of configure and if needed change installation dir by --bashcompletionsdir=DIR option. There was another suggestion to keep bash-completion installation enabled by default and make it warn instead of error. So I am open for votes for both suggestions or may be something else.