fix some lints caught by Clippy

Most of these changes are very simple. Among the changes made involve
taking advantage of auto-deref (`(*val).foo()` -> `val.foo()`) and
removing instances where we create a ref and immediately dereference it
(`&*val` -> `val`). There was a pretty neat case in `posix_openpt` where
some pointer verbosity was able to be reduced by using the more modern C
strings rather than the byte strings with an explicit NUL at the end.

Additionally, `exit()` now calls `unreachable!()` at the end. We
previously did `loop {}`, but clippy didn't like this. It can be up for
debate whether we want to make this `unreachable_unchecked` or similar.

There is only one change that might cause any sort of concern, and that
is the change from `.skip_while(!p).next()` -> `.find(p)`. This, like
everything else, was caught in a Clippy lint but I believe it deserves
some explanation because it isn't immediately obvious. Info about the
lint is here: https://rust-lang.github.io/rust-clippy/rust-1.89.0/index.html#skip_while_next

src/header/stdio/mod.rs

@@ -1275,8 +1275,7 @@ pub unsafe extern "C" fn tempnam(dir: *const c_char, pfx: *const c_char) -> *mut
         [tmpdir, dir, P_tmpdir.as_ptr().cast()]
             .iter()
             .copied()
-            .skip_while(|&d| !unsafe { is_appropriate(d) })
-            .next()
+            .find(|&d| unsafe { is_appropriate(d) })
             .unwrap_or(c"/tmp".as_ptr().cast())
     };
     let dirname_len = unsafe { string::strlen(dirname) };