shell: modules: Add date commands to display or set date

Currently only supports Unix time.

‘get’ subcommand returns date in format “Y-m-d H:M:S”
‘set’ subcommand format is ‘[Y-m-d] <H:M:S>’

The ‘set’ subcommand is implemented with basic date validation.

For user convenience of small adjustments to time the time argument
will accept H:M:S, :M:S or ::S where the missing field(s) will be
filled in by the previous time state.

Signed-off-by: Nick Ward <nix.ward@gmail.com>