KGDB/KDB fixes and cleanups

 Cleanups
   kdb: Remove unused command flags, repeat flags and KDB_REPEAT_NONE

 Fixes
   kgdb/kdb: Allow access on a single core, if a CPU round up is deemed
      impossible, which will allow inspection of the now "trashed" kernel
   kdb: Add enable mask for the command groups
   kdb: access controls to restrict sensitive commands
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