touch: Support custom timestamps

Previously, `touch` remained limited to creating files and updating
their current access and modifications time to the current time. It's
now capable of accepting two different timestamp formats with flags `-d`
and `-t` and referencing timestamps of other files with flag `-r`.

`touch` can also update only the last access time with `-a`, only the
last access time with `-m`, or update both as usual. `touch` updates
both left unspecified.

With `-c`, `touch` does not create a file if it doesn't already exist.

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