Kernel/LibC: Implement realpath

realpath is implemented as a syscall. This is not really required but it
was the easiest way to get it working as there is already path
canonicalization at kernel level.

userspace/libraries/LibC/include/sys/syscall.h

@@ -81,6 +81,7 @@ __BEGIN_DECLS
 	O(SYS_GETSOCKNAME,		getsockname)	\
 	O(SYS_GETSOCKOPT,		getsockopt)		\
 	O(SYS_SETSOCKOPT,		setsockopt)		\
+	O(SYS_REALPATH,			realpath)		\
 
 enum Syscall
 {