ixgbe: sysctl for TCP flag handling during TSO

Add tso_tcp_flags_mask_first_segment, tso_tcp_flags_mask_middle_segment,
and tso_tcp_flags_mask_last_segment sysctl-variables to control the
handling of TCP flags during TSO.

This allows to fix the masks appropriate for classical ECN and to
configure appropriate masks for accurate ECN.

Michael notes emperically 82599 has an unexpected middle mask:
Chip First Middle Last
82599 0xFF6 0xFF6 0xF7F

which should be fixed up to 0xF76 (RFC 3168) in a future commit.

Reviewed by: rrs, rscheff
MFC after: 3 days
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D44258