libc: Add getdtablesize()

No system call for this, since the limit is fixed right now

libs/libc/include/unistd.h

@@ -103,6 +103,9 @@ extern "C"
     int chdir(const char* path);  // Not implemented.
     int pipe(int fd[2]);          // Not implemented.
 
+    /* Returns the maximum number of file descriptors a process can have open at the same time. */
+    int getdtablesize(void);
+
 #ifdef __cplusplus
 }
 #endif

libs/libc/src/unistd.cpp

@@ -163,4 +163,9 @@ extern "C"
     {
         NOT_IMPLEMENTED("pipe");
     }
+
+    int getdtablesize(void)
+    {
+        return OPEN_MAX;
+    }
 }