Kernel/LibC: Implement SYS_ISATTY and isatty()

libc/include/sys/syscall.h

@@ -77,6 +77,7 @@ __BEGIN_DECLS
 	O(SYS_SMO_CREATE,		smo_create)		\
 	O(SYS_SMO_DELETE,		smo_delete)		\
 	O(SYS_SMO_MAP,			smo_map)		\
+	O(SYS_ISATTY,			isatty)			\
 
 enum Syscall
 {

libc/unistd.cpp

@@ -114,6 +114,11 @@ int dup2(int fildes, int fildes2)
 	return syscall(SYS_DUP2, fildes, fildes2);
 }
 
+int isatty(int fildes)
+{
+	return syscall(SYS_ISATTY, fildes) >= 0;
+}
+
 int execl(const char* pathname, const char* arg0, ...)
 {
 	if (arg0 == nullptr)