LibC: Implement pthread cancelation

This code is not tested at all but it looks correct xD

userspace/libraries/LibC/fcntl.cpp

@@ -1,15 +1,19 @@
 #include <fcntl.h>
+#include <pthread.h>
 #include <stdarg.h>
 #include <sys/syscall.h>
 #include <unistd.h>
 
 int creat(const char* path, mode_t mode)
 {
+	pthread_testcancel();
 	return open(path, O_WRONLY | O_CREAT | O_TRUNC, mode);
 }
 
 int open(const char* path, int oflag, ...)
 {
+	pthread_testcancel();
+
 	va_list args;
 	va_start(args, oflag);
 	mode_t mode = va_arg(args, mode_t);
@@ -20,6 +24,8 @@ int open(const char* path, int oflag, ...)
 
 int openat(int fd, const char* path, int oflag, ...)
 {
+	pthread_testcancel();
+
 	va_list args;
 	va_start(args, oflag);
 	mode_t mode = va_arg(args, mode_t);
@@ -30,6 +36,8 @@ int openat(int fd, const char* path, int oflag, ...)
 
 int fcntl(int fildes, int cmd, ...)
 {
+	pthread_testcancel();
+
 	va_list args;
 	va_start(args, cmd);
 	int extra = va_arg(args, int);