gnunet's config section may contain dashes '-' which isn't an
allowed character for sections in UCI.
Thus replace the first underscore with a dash which in gnunet-config
sections, as that happends to be work for all currently used sections.
e.g. this now allows accessing sections like transport-http_server via
an UCI sections called transport_http_server as well as namestore-flat
using an UCI section called namestore_flat.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>