liboconfig: Include the filename in error messages.

liboconfig: Include the filename in error messages.

As collectd now supports more than one config file, this is more
convenient.

A module-global variable is used for that purpose. If no filename is
available (e.g. if the user uses oconfig_parse_fh() directly), a string
like "<fd#X>" is used instead, where X is replaced by the file descriptor.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>