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.

Base/usr/share/man/man2/geteuid.md

@@ -7,8 +7,8 @@ geteuid, getegid - get effective user / group id
 ```**c++
 #include <unistd.h>
 
-uid_t geteuid(void);
-gid_t getegid(void);
+uid_t geteuid();
+gid_t getegid();
 ```
 
 ## Description

Base/usr/share/man/man2/getuid.md

@@ -7,8 +7,8 @@ getuid, getgid - get real user / group id
 ```**c++
 #include <unistd.h>
 
-uid_t getuid(void);
-gid_t getgid(void);
+uid_t getuid();
+gid_t getgid();
 ```
 
 ## Description