ddb: Rework macros to make it easier to add new command tables.

• Add new DB_DEFINE_TABLE and DB_DECLARE_TABLE macros to define new command tables. DB_DECLARE_TABLE is intended for use in headers similar to MALLOC_DECLARE and SYSCTL_DECL.
  
  DB_DEFINE_TABLE takes three arguments, the name of the parent table, the command name, and the name of the table itself, e.g. DB_DEFINE_TABLE(show, foo, show_foo) defines a new "show foo" table.

• DB_TABLE_COMMAND, DB_TABLE_COMMAND_FLAGS, DB_TABLE_ALIAS, and DB_ALIAS_FLAGS allow new commands and aliases to be defined. These are similar to the existing DB_COMMAND, etc. except that they take an initial argument giving the name of the parent table, e.g.:
  
  DB_TABLE_COMMAND(show_foo, bar, db_show_foo_bar)
  
  defines a new "show foo bar" command.

This provides a cleaner interface than the ad-hoc use of internal
macros like _DB_SET that was required previously (e.g. in cxgbe(4)).

This retires DB_FUNC macro as well as the internal _DB_FUNC macro.

Reviewed by: melifaro, kib, markj
Differential Revision: https://reviews.freebsd.org/D40819