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/calls/chdir.c

@@ -29,7 +29,8 @@
 /**
  * Sets current directory.
  *
- * This does *not* update the `PWD` environment variable.
+ * The current directory is shared by all threads in a process. This
+ * does not update the `PWD` environment variable.
  *
  * @return 0 on success, or -1 w/ errno
  * @raise ELOOP if a loop was detected resolving components of `path`