ddb: namespacing of struct command

'command' is too generic for something specific to the kernel debugger;
change this so it is less likely to collide with local variable names.
Also rename struct command_table to struct db_command_table.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D35367