Kernel: Remove SYS_DUP and implement it using fcntl F_DUPFD

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

@@ -36,7 +36,6 @@ __BEGIN_DECLS
 	O(SYS_SET_PWD,			setpwd)			\
 	O(SYS_CLOCK_GETTIME,	clock_gettime)	\
 	O(SYS_PIPE,				pipe)			\
-	O(SYS_DUP,				dup)			\
 	O(SYS_DUP2,				dup2)			\
 	O(SYS_KILL,				kill)			\
 	O(SYS_TCGETPGRP,		tcgetpgrp)		\

userspace/libraries/LibC/unistd.cpp

@@ -125,7 +125,7 @@ int fsync(int fildes)
 
 int dup(int fildes)
 {
-	return syscall(SYS_DUP, fildes);
+	return fcntl(fildes, F_DUPFD, 0);
 }
 
 int dup2(int fildes, int fildes2)