bootstrap: sanity check install prefix and sysconfdir

[cuviper]

In the followup discussion on #110906, we discovered that their original problem was because they had chosen a path under their home directory for [install] prefix, but they had not changed the default sysconfdir = "/etc". So most things installed fine, but they didn't have permission to write /etc/bash_completion.d/cargo.

Bootstrap could sanity check this when starting an install, seeing that prefix is writable by the current user but the absolute sysconfdir is not. (A relative sysconfdir goes under prefix, which should be fine.) This should also take DESTDIR into account as a further prefix on all installation paths.

[LJason77]

I have just updated to version 1.71.0, and now I'm receiving the terminal warning:
~/.rustup/toolchains/stable-aarch64-unknown-linux-gnu/etc/bash_completion.d/cargo: No such file or directory

In my previous setup, I added the line source $(rustc --print sysroot)/etc/bash_completion.d/cargo to my ~/.bashrc file.

[cuviper]

@LJason77 yeah, #110906 made that effectively $sysroot/src/etc. That was reverted in #113579, and now proposed for stable and beta backports. So you could add src/ for now, but it will be going back to the way it was.