Fix default value conflicts for uIP and sicslowpan config params

#178 (2d50a40) introduced contiki-default-conf.h, which specifies default values for various configuration parameters. Among those are some parameters for uIP and sicslowpan.

As a result, some parameters now have defaults specified in two places, with contiki-default-conf.h changing previous defaults. For instance, `UIP_CONS` was changed from 10 to 8 and the default for `SICSLOWPAN_CONF_FRAG` was changed from 0 to 1.

These patches tidy up the situation: Defaults for UIP_xyz are now only specified in uipopt.h. Defaults for SICSLOWPAN_xyz are now only specified in contiki-default-conf.h

For UIP_xyz, old defaults from uipopt.h have been restored. For SICSLOWPAN_xyz, new defaults specified in contiki-default-conf.h were retained.