Kernel: Pass characters+length to readlink()

Note that I'm developing some helper types in the Syscall namespace as
I go here. Once I settle on some nice types, I will convert all the
other syscalls to use them as well.

Kernel/Syscall.h

@@ -305,6 +305,12 @@ struct SyscallString {
     size_t length { 0 };
 };
 
+template<typename DataType, typename SizeType>
+struct MutableBufferArgument {
+    DataType* data { nullptr };
+    SizeType size { 0 };
+};
+
 struct SyscallStringList {
     SyscallString* strings { nullptr };
     size_t length { 0 };
@@ -316,6 +322,11 @@ struct SC_execve_params {
     SyscallStringList environment;
 };
 
+struct SC_readlink_params {
+    SyscallString path;
+    MutableBufferArgument<char, size_t> buffer;
+};
+
 void initialize();
 int sync();