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.

Userland/allocate.cpp

@@ -31,7 +31,7 @@
 #include <string.h>
 #include <unistd.h>
 
-static void usage(void)
+static void usage()
 {
     printf("usage: allocate [number [unit (B/KiB/MiB)]]\n");
     exit(1);