Introduce ERRNO_IS_FS_WRITE_REFUSED(), and use it in binfmt_mounted() (#38117)

- This introduces ERRNO_IS_FS_WRITE_REFUSED(), and apply it where
usable.
- This makes unexpected errors in access_fd() called by binfmt_mounted()
propagated to the caller.
- Renames binfmt_mounted() to binfmt_mounted_and_writable(), as it also
checks the fs is writable.
- Voidifies one disable_binfmt() call in shutdown.c.

diff --cc src/nspawn/nspawn.c
Simple merge