libc/db: Sync with FreeBSD

- libc/db/hash: cap auto-tuned block size with a value that actually
  works
- Consider flag == 0 as the same of flag == R_NEXT
- Minor changes from Berkeley DB 1.86 and further improvements from
  OpenBSD
- Allow O_SYNC and O_NOFOLLOW flags in dbopen()
- Plug memory leaks and a potential NULL dereference
- Simplify the logic when determining whether to zero out a db file to
  after open().
- When deleting a big key, the offset of an empty page should be bsize,
  not bsize-1
- Avoid overwriting the cursor page when the cursor page becomes the LRU
  page.
- Fix for sequential retrieval failure when using large key/data pairs.
- Fix a crash when iterating over a hash and removing its elements.
- Only squeeze a short key/value pair onto a page with other complete
  key/value pairs
- Return meaningful errno in overflow case
- Save errno before calling _close(), which may clear it
- Explicitly specify bit width for on-disk data structure.

Obtained-from:   FreeBSD
boot - Add loader.conf variable sioX.baud for console operation

* We already have 'comconsole' and 'sioX.flags=0x10' to set the
  serial port to use as the console, which the kernel boot uses.
  Add in 'sioX.baud' to set the baud rate, so we don't have to
  compile a custom kernel.

* Useful for IPMI setups.
boot - Clarify loader.conf documentation

* Add clarification for situations where you want the boot loader
  to use video output for IPMI purposes, but you want the kernel to
  use sio1.