Fixes #10578

The previous check only worked if the user could run `sudo` without typing the
password, which is almost none (I checked in Google Cloud Shell so I failed to
notice this).

This new check works whether the user has no sudo privileges, or if it has,
whether they have to type in the password or not.

It should really be easier to check if the user doesn't have privilege without
having to make them type the password.

This fixes the error in Google Cloud Shell, where a password prompt
appears when running `chsh` but the user (hello) does not have a
password.

If ran with `sudo`, the `chsh` command happens without a password
prompt.

When generating completions for Cargo, if the Rust sysroot
(i.e. `rustc +${${(z)$(rustup default)}[1]} --print sysroot`) contains
spaces, Cargo completions will not work because the spaces are not
escaped, thus passing two arguments to the "source" command instead of
one. The spaces need to be escaped for this to work.

By default, `typeset` defines variables locally unless in the main scope.
This is specially bad when using `omz plugin load`, which happens inside
a function, so the declared variables don't continue being defined when
the function finishes and the main scope reappears.

Fixes #10562

Fixes #10560

Also cleaned up the code a bit

Fixes #10561