Kernel/LibC: Implement SA_RESTART

I have been thinking how to do this for a long time but I finally
figured out a semi-clean way

userspace/libraries/LibC/include/errno.h

@@ -90,6 +90,7 @@ __BEGIN_DECLS
 #define EXDEV			81
 #define ENOTBLK			82
 
+#define ERESTART        0xFE /* internal errno for SA_RESTART */
 #define EUNKNOWN		0xFF
 
 #define errno (*__errno_location())

userspace/libraries/LibC/unistd.cpp

@@ -83,7 +83,10 @@ long syscall(long syscall, ...)
 
 	va_end(args);
 
-	long ret = Kernel::syscall(syscall, arg1, arg2, arg3, arg4, arg5);
+	long ret;
+	do
+		ret = Kernel::syscall(syscall, arg1, arg2, arg3, arg4, arg5);
+	while (ret == -ERESTART);
 
 	if (ret < 0)
 	{