Use standard logging (FIB_XX_LOG) across nhg code instead of using old-style DPRINTFs.
Add debug object printer for nhgs (`nhgrp_print_buf`).
Example:
```
Jun 19 20:17:09 devel2 kernel: [nhgrp] inet.0 nhgrp_ctl_alloc_default: multipath init done
Jun 19 20:17:09 devel2 kernel: [nhg_ctl] inet.0 alloc_nhgrp: num_nhops: 2, compiled_nhop: 2
Jun 19 20:17:26 devel2 kernel: [nhg_ctl] inet.0 alloc_nhgrp: num_nhops: 3, compiled_nhop: 3
Jun 19 20:17:26 devel2 kernel: [nhg_ctl] inet.0 destroy_nhgrp: destroying nhg#0/sz=2:[#6:1,#5:1]
```