Get GNU GMP test suite fully passing

- Fix stdio fmemopen() buffer behaviors
- Fix scanf() to return EOF when appropriate
- Prefer fseek/ftell names over fseeko/ftello
- Ensure locale field is always set in the TIB
- Fix recent regression in vfprintf() return count
- Make %n directive in scanf() have standard behavior

libc/stdio/fseek_unlocked.c

@@ -0,0 +1,83 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2021 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/calls/calls.h"
+#include "libc/errno.h"
+#include "libc/macros.internal.h"
+#include "libc/stdio/internal.h"
+#include "libc/stdio/stdio.h"
+#include "libc/sysv/consts/o.h"
+
+/**
+ * Repositions open file stream.
+ *
+ * This function flushes the buffer (unless it's currently in the EOF
+ * state) and then calls lseek() on the underlying file. If the stream
+ * is in the EOF state, this function can be used to restore it without
+ * needing to reopen the file.
+ *
+ * @param f is a non-null stream handle
+ * @param offset is the byte delta
+ * @param whence can be SEET_SET, SEEK_CUR, or SEEK_END
+ * @returns 0 on success or -1 on error
+ */
+int fseek_unlocked(FILE *f, int64_t offset, int whence) {
+  int res;
+  ssize_t rc;
+  int64_t pos;
+  if (f->fd != -1) {
+    if (__fflush_impl(f) == -1) return -1;
+    if (whence == SEEK_CUR && f->beg < f->end) {
+      offset -= f->end - f->beg;
+    }
+    if (lseek(f->fd, offset, whence) != -1) {
+      f->beg = 0;
+      f->end = 0;
+      f->state = 0;
+      res = 0;
+    } else {
+      f->state = errno == ESPIPE ? EBADF : errno;
+      res = -1;
+    }
+  } else {
+    switch (whence) {
+      case SEEK_SET:
+        pos = offset;
+        break;
+      case SEEK_CUR:
+        pos = f->beg + offset;
+        break;
+      case SEEK_END:
+        pos = f->end + offset;
+        break;
+      default:
+        pos = -1;
+        break;
+    }
+    f->end = MAX(f->beg, f->end);
+    if (0 <= pos && pos <= f->end) {
+      f->beg = pos;
+      f->state = 0;
+      res = 0;
+    } else {
+      f->state = errno = EINVAL;
+      res = -1;
+    }
+  }
+  return res;
+}