ndomod: Cleanup config file and runtime var dumping.

Replace our last direct uses of asprintf() in ndomod with ndo_dbuf
equivalents which check for failed allocation.
ndo_dbuf_printf: Make NULL buf case explicit and clearer.
ndomod: Only nebmodule_(de)init() need external linkage.

All other ndomod_* functions in ndomod.[hc] can/should have internal
linkage (static). Leave the declatations in ndomod.h since only
nodmod.c includes it.

Refactor some comments, spacing/indentation, and overly verbose
functions. Add conditionally compiled opbject dump timing.
ndomod: Pad usecs to six digits when printing timevals.
ndomod: ARRAY_SIZE not sizeof for element count.