db_command.c: use designated initializers

[mhorne]

Summary

Provide separate helper macros for regular commands and next-level table
commands as they are mutually exclusive. This ensures proper
initialization of each element and allows us to exclude some redundant
fields, such as specifying .more = NULL for every regular command.

[jhb]

sys/ddb/db_command.c

89

I would perhaps call this DB_TABLE_CMD or DB_TABLE