Everywhere: Switch from (void) to [[maybe_unused]] (#4473)

Problem:
- `(void)` simply casts the expression to void. This is understood to
  indicate that it is ignored, but this is really a compiler trick to
  get the compiler to not generate a warning.

Solution:
- Use the `[[maybe_unused]]` attribute to indicate the value is unused.

Note:
- Functions taking a `(void)` argument list have also been changed to
  `()` because this is not needed and shows up in the same grep
  command.

Services/TelnetServer/main.cpp

@@ -57,13 +57,13 @@ static void run_command(int ptm_fd, String command)
         }
 
         // NOTE: It's okay if this fails.
-        (void)ioctl(0, TIOCNOTTY);
+        [[maybe_unused]] auto rc = ioctl(0, TIOCNOTTY);
 
         close(0);
         close(1);
         close(2);
 
-        int rc = dup2(pts_fd, 0);
+        rc = dup2(pts_fd, 0);
         if (rc < 0) {
             perror("dup2");
             exit(1);