Merge topic 'cmFileCommand-refactoring'

a7a5f376bc cmFileCommand: Use cmSubcommandTable
b66b7464ab Introduce cmSubcommandTable
2b785875fb cmFileCommand: turn into free function
9703c65718 cmFileCommand: put subcommands in unnamed namespace
64f987c174 cmFileCommand: port to cmExecutionStatus

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !3657