to_flags is currently a 64-bit integer; however, we only use 7 bits.
Furthermore, there is no reason this needs to be a 64-bit integer
for the forseeable future.

Also, there is an inconsistency between to_flags and the mask in
tcp_addoptions(). Before r195654, to_flags was a u_long and the mask in
tcp_addoptions() was a u_int. r195654 changed to_flags to be a u_int64_t
but left the mask in tcp_addoptions() as a u_int, meaning that these
variables will only be the same width on platforms with 64-bit integers.

Convert both to_flags and the mask in tcp_addoptions() to be explicitly
32-bit variables. This may save a few cycles on 32-bit platforms, and
avoids unnecessarily mixing types.

Differential Revision: https://reviews.freebsd.org/D5584
Reviewed by: hiren
MFC after: 2 weeks
Sponsored by: Juniper Networks