Switch options and mode bits to uint32_t from unsigned long.

This is a bitmask, so the number of bits available should be the same
across all platforms.

Change-Id: I98e8d375fc7d042aeae1270174bc8fc63fba5dfc
Reviewed-on: https://boringssl-review.googlesource.com/4556
Reviewed-by: Adam Langley <agl@google.com>