mktemp: simplify file path parameter logic

Simplify the logic of computing the file path parameters (the
directory, prefix, suffix, and number of random characters) for the
temporary file created by `mktemp`. This commits adds an `Options`
struct as a layer of indirection between the application logic and
`clap`, and a `Params` struct whose associated function is responsible
for determining the file path parameters from the `Options`. This is
an improvement because the previous code had some logic for
determining file path parameters in one place and some in another
place.