Get sudo-rs to compile on FreeBSD

[squell]

You are allowed to cut corners here, e.g. disable some parts (preferably not too much).

This PR has a few easy intermediate steps:

• sudo-rs compiles but doesn't actually work (i.e. crashes or does the wrong thing)
• sudo-rs compiles and prints the help accurately
• sudo-rs is able to launch a process (without any feature we actually care about such as authenticating the user properly, setting the timestamps, and so on)

Getting PAM and any other system interface working properly is a separate issue. The goal of this PR is to get a baseline.

Needless to say, this should not result in a PR on our current main, but rather something that lives in a (longer-lived) branch.

Don't forget to re-introduce the "Using this version of sudo-rs is likely to break your system" SUDO_RS_IS_UNSTABLE feature for this branch (or similar precaution)

[squell]

Note, if #862 is merged we already have a temporary "foolproof" since sudo wil refuse operation on FreeBSD.

[bjorn3]

The existing kernel_check already causes sudo-rs to refuse to run on FreeBSD.