Fix issues with stdio needed for Lua

See #61

libc/stdio/ftell.c

@@ -17,6 +17,7 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/calls/calls.h"
+#include "libc/errno.h"
 #include "libc/stdio/stdio.h"
 
 /**
@@ -25,6 +26,23 @@
  * @param stream is a non-null stream handle
  * @returns current byte offset from beginning of file, or -1
  */
-long ftell(FILE *stream) {
-  return fseek(stream, 0, SEEK_CUR);
+long ftell(FILE *f) {
+  int64_t pos;
+  if (f->fd != -1) {
+    if (f->beg && !f->end) {
+      f->writer(f);
+    }
+    if ((pos = lseek(f->fd, 0, SEEK_CUR)) != -1) {
+      f->state = 0;
+      f->beg = 0;
+      f->end = 0;
+      return pos;
+    } else {
+      f->state = errno == ESPIPE ? EBADF : errno;
+      return -1;
+    }
+  } else {
+    errno = f->state;
+    return -1;
+  }
 }