Kernel/LibC: Implement {get,set}sockopt()

These are pretty much dummy functions in the kernel side. Only case that
is handled is SOL_SOCKET with SO_ERROR. This is hard coded to return no
error. Network stack is currently synchronous, so all errors are already
reported through synchronous network functions.

libc/include/sys/syscall.h

@@ -79,6 +79,8 @@ __BEGIN_DECLS
 	O(SYS_SMO_MAP,			smo_map)		\
 	O(SYS_ISATTY,			isatty)			\
 	O(SYS_GETSOCKNAME,		getsockname)	\
+	O(SYS_GETSOCKOPT,		getsockopt)		\
+	O(SYS_SETSOCKOPT,		setsockopt)		\
 
 enum Syscall
 {