Kernel: Use the Syscall string and buffer types more

While I was updating syscalls to stop passing null-terminated strings,
I added some helpful struct types:

    - StringArgument { const char*; size_t; }
    - ImmutableBuffer<Data, Size> { const Data*; Size; }
    - MutableBuffer<Data, Size> { Data*; Size; }

The Process class has some convenience functions for validating and
optionally extracting the contents from these structs:

    - get_syscall_path_argument(StringArgument)
    - validate_and_copy_string_from_user(StringArgument)
    - validate(ImmutableBuffer)
    - validate(MutableBuffer)

There's still so much code around this and I'm wondering if we should
generate most of it instead. Possible nice little project.

Libraries/LibC/stdlib.cpp

@@ -711,7 +711,7 @@ char* realpath(const char* pathname, char* buffer)
     size_t size = PATH_MAX;
     if (buffer == nullptr)
         buffer = (char*)malloc(size);
-    Syscall::SC_realpath_params params { pathname, strlen(pathname), buffer, size };
+    Syscall::SC_realpath_params params { { pathname, strlen(pathname) }, { buffer, size } };
     int rc = syscall(SC_realpath, &params);
     if (rc < 0) {
         errno = -rc;