BT_LOG_WRITE_ERRNO(): use `_msg ": %s" _fmt` format

BT_LOG_WRITE_ERRNO(): use `_msg ": %s" _fmt` format

Do not put a period at the end of the errno string: this gives the
opportunity to the user to follow the errno string with a colon, a
space, and fields, e.g.:

    BT_LOGE_ERRNO("Cannot write to file", ": filename=\"%s\"", filename);

which would print this message, for example:

    Cannot write to file: No space left on device: filename="/tmp/my-file"

You can also terminate a field-less message with a period manually:

    BT_LOGE_ERRNO("Cannot close file", ".");

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>