Cleaned up & fixed buffer alignment handling.

- Fixed ForceVectorAlignment (and possibly other call-sites) not
  setting minalign_.
- Fixed flipped alignment parameters in CopyTable (reflection).
- Made aligment for FlatBufferBuilder internal buffer configurable
  (useful when reading a constructed buffer directly).
- Ensured Alignment rounding is always up.

Change-Id: I33ca4887d92a09cb11a369c14a109f4b07ae707a
Tested: on Linux.