v.pref: cleanup os from and to string functions

[ttytm]

Split up of #21395 to allow isolating bug.

@spytheman in regards to what I commented in the linked PR about using:

fn res_fn()  ! {
	return match {
		// ...
		else { error() }
		// vs
		else { return error() }
	}
}

I run the changes with just error instead of the return error in os_from_string with the results: https://github.com/ttytm/v/actions?query=branch%3Apriv%2Fpref%2Fos-cleanup+