config: provide access to the current line number

config: provide access to the current line number

Users are nowadays trained to see message from CLI tools in the form

    <file>:<lno>: …

To be able to give such messages when notifying the user about
configurations in any config file, it is currently only possible to get
the file name (if the value originates from a file to begin with) via
`current_config_name()`. Now it is also possible to query the current line
number for the configuration.

Signed-off-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>