Introduce Variants of Some Functions that Take Optional File Descriptors

Introduce Variants of Some Functions that Take Optional File Descriptors

Somewhat surprisingly, calling `fchmod()` is non-trivially faster than calling
`chmod()`, and so on.

This commit introduces alternate variants to some common file util functions
that take an optional fd. If present, they call the `f`-variant of the
underlying function. Otherwise, they fall back to the regular filename-taking
version of the function.